fix: Drop unnecessary unique index on vulnerability_signatures

What does this MR do and why?

fix: Drop unnecessary unique index on vulnerability_signatures

Unique index is unneeded and causes violations when vulnerabilities may have multiple signatures.

There is already a unique index on idx_vuln_signatures_uniqueness_signature_sha, so while this fixes the underlying bug a relevant uniqueness constraint continues to remain in place.

Relates to #419737 (closed)

Migration logs

db:migrate

ci: == [advisory_lock_connection] object_id: 242000, pg_backend_pid: 64896
main: == [advisory_lock_connection] object_id: 255080, pg_backend_pid: 64899
main: == 20230727203840 DropUniqueIdxOnVulnSignatures: migrating ====================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0007s
main: -- indexes(:vulnerability_finding_signatures)
main:    -> 0.0071s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- remove_index(:vulnerability_finding_signatures, {:algorithm=>:concurrently, :name=>"idx_vuln_signatures_on_occurrences_id_and_signature_sha"})
main:    -> 0.0095s
main: -- execute("RESET statement_timeout")
main:    -> 0.0005s
main: == 20230727203840 DropUniqueIdxOnVulnSignatures: migrated (0.0363s) ===========

main: == [advisory_lock_connection] object_id: 255080, pg_backend_pid: 64899
ci: == [advisory_lock_connection] object_id: 268720, pg_backend_pid: 64901
ci: == 20230727203840 DropUniqueIdxOnVulnSignatures: migrating ====================
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0008s
ci: -- indexes(:vulnerability_finding_signatures)
ci:    -> 0.0059s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0003s
ci: -- remove_index(:vulnerability_finding_signatures, {:algorithm=>:concurrently, :name=>"idx_vuln_signatures_on_occurrences_id_and_signature_sha"})
ci:    -> 0.0043s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0008s
ci: == 20230727203840 DropUniqueIdxOnVulnSignatures: migrated (0.0395s) ===========

ci: == [advisory_lock_connection] object_id: 268720, pg_backend_pid: 64901
~/code/gl/gdk/gitlab drop-unique-signatures-idx* 18s

How to set up and validate locally

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MR acceptance checklist

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