Drop `index_vulnerability_occurrences_for_issue_links_migration`

What does this MR do and why?

This index was added in 13.3 to support a data migration in Add migration helper index for Vulnerabilities:... (!38898 - merged) • Tiger Watson • 13.3

Verification that index is unused

The "Finding unused indexes" docs say that you can check if an index is still used in production using the following thanos query:

sum by (type)(rate(pg_stat_user_indexes_idx_scan{env="gprd", indexrelname="index_vulnerability_occurrences_for_issue_links_migration"}[180d]))

doing so results in no usage:

thanos results

thanos results

thanos deprecation

There is a warning about thanos being deprecated, so perhaps I need to check grafana instead

I ran a similar query in grafana and also see no usages

sum by(type) (rate(pg_stat_user_indexes_idx_scan{indexrelname="index_vulnerability_occurrences_for_issue_links_migration"}[30d]))

grafana results

Migration output

bundle exec rails db:migrate

main: == [advisory_lock_connection] object_id: 129000, pg_backend_pid: 329519
main: == 20240709093324 DropIndexVulnerabilityOccurrencesForIssueLinksMigration: migrating 
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0289s
main: -- indexes(:vulnerability_occurrences)
main:    -> 0.0056s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0002s
main: -- remove_index(:vulnerability_occurrences, {:algorithm=>:concurrently, :name=>"index_vulnerability_occurrences_for_issue_links_migration"})
main:    -> 0.0034s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20240709093324 DropIndexVulnerabilityOccurrencesForIssueLinksMigration: migrated (0.0482s) 

main: == [advisory_lock_connection] object_id: 129000, pg_backend_pid: 329519
ci: == [advisory_lock_connection] object_id: 129220, pg_backend_pid: 329521
ci: == 20240709093324 DropIndexVulnerabilityOccurrencesForIssueLinksMigration: migrating 
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0006s
ci: -- indexes(:vulnerability_occurrences)
ci:    -> 0.0047s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0002s
ci: -- remove_index(:vulnerability_occurrences, {:algorithm=>:concurrently, :name=>"index_vulnerability_occurrences_for_issue_links_migration"})
ci:    -> 0.0027s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0001s
ci: == 20240709093324 DropIndexVulnerabilityOccurrencesForIssueLinksMigration: migrated (0.0197s) 

ci: == [advisory_lock_connection] object_id: 129220, pg_backend_pid: 329521

bundle exec rails db:migrate:down:main VERSION=20240709093324

main: == [advisory_lock_connection] object_id: 128500, pg_backend_pid: 334691
main: == 20240709093324 DropIndexVulnerabilityOccurrencesForIssueLinksMigration: reverting 
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0101s
main: -- index_exists?(:vulnerability_occurrences, "project_id, report_type, encode(project_fingerprint, 'hex'::text)", {:name=>"index_vulnerability_occurrences_for_issue_links_migration", :algorithm=>:concurrently})
main:    -> 0.0044s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0002s
main: -- add_index(:vulnerability_occurrences, "project_id, report_type, encode(project_fingerprint, 'hex'::text)", {:name=>"index_vulnerability_occurrences_for_issue_links_migration", :algorithm=>:concurrently})
main:    -> 0.0101s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20240709093324 DropIndexVulnerabilityOccurrencesForIssueLinksMigration: reverted (0.0381s) 

main: == [advisory_lock_connection] object_id: 128500, pg_backend_pid: 334691

bundle exec rails db:migrate:down:ci VERSION=20240709093324

ci: == [advisory_lock_connection] object_id: 128500, pg_backend_pid: 335058
ci: == 20240709093324 DropIndexVulnerabilityOccurrencesForIssueLinksMigration: reverting 
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0025s
ci: -- index_exists?(:vulnerability_occurrences, "project_id, report_type, encode(project_fingerprint, 'hex'::text)", {:name=>"index_vulnerability_occurrences_for_issue_links_migration", :algorithm=>:concurrently})
ci:    -> 0.0048s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0002s
ci: -- add_index(:vulnerability_occurrences, "project_id, report_type, encode(project_fingerprint, 'hex'::text)", {:name=>"index_vulnerability_occurrences_for_issue_links_migration", :algorithm=>:concurrently})
ci:    -> 0.0157s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0006s
ci: == 20240709093324 DropIndexVulnerabilityOccurrencesForIssueLinksMigration: reverted (0.0430s) 

ci: == [advisory_lock_connection] object_id: 128500, pg_backend_pid: 335058

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resolves: #471153
Changelog: removed