Remove plaintext token index from ci_builds

What does this MR do and why?

Related to #339191 (closed)

This was dropped in production via change request gitlab-com/gl-infra/production#7345 (comment 1030499429) and this MR should be a no-op on .com.

The ci_builds token column is no longer used and we're in the process of removing it #339192 (closed), but it has an index attached to it. Since this is a high traffic table, it might be a good idea to drop the index in a separate migration.

List of relations  
 Schema |               Name               | Type  |  Owner  |   Table   |  Size   | Description   
--------+----------------------------------+-------+---------+-----------+---------+-------------  
 public | index_ci_builds_on_token_partial | index | joe_309 | ci_builds | 4421 MB |   
(1 row) 

Screenshots or screen recordings

https://thanos.gitlab.net/graph?g0.expr=sum(rate(pg_stat_user_indexes_idx_tup_read%7Benv%3D%22gprd%22%2C%20indexrelname%3D%22index_ci_builds_on_token_partial%22%7D%5B5m%5D))&g0.tab=0&g0.stacked=0&g0.range_input=2w&g0.max_source_resolution=0s&g0.deduplicate=1&g0.partial_response=0&g0.store_matches=%5B%5D&g1.expr=sum(rate(pg_stat_user_indexes_idx_tup_fetch%7Benv%3D%22gprd%22%2C%20indexrelname%3D%22index_ci_builds_on_token_partial%22%7D%5B5m%5D))&g1.tab=0&g1.stacked=0&g1.range_input=2w&g1.max_source_resolution=0s&g1.deduplicate=1&g1.partial_response=0&g1.store_matches=%5B%5D&g2.expr=sum(rate(pg_stat_user_indexes_idx_scan%7Benv%3D%22gprd%22%2C%20indexrelname%3D%22index_ci_builds_on_token_partial%22%7D%5B5m%5D))&g2.tab=0&g2.stacked=0&g2.range_input=2w&g2.max_source_resolution=0s&g2.deduplicate=1&g2.partial_response=0&g2.store_matches=%5B%5D

Migrations output:

marius@roast-carrot ~/W/g/gitlab (339191-remove-plaintext-token-index-from-ci-builds)> VERSION=20220628111752 bin/rails db:migrate:up:ci
ci: == 20220628111752 DropTokenIndexFromCiBuilds: migrating =======================
ci: -- transaction_open?()
ci:    -> 0.0001s
ci: -- indexes(:ci_builds)
ci:    -> 0.0274s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0003s
ci: -- remove_index(:ci_builds, {:algorithm=>:concurrently, :name=>"index_ci_builds_on_token_partial"})
ci:    -> 0.0091s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0004s
ci: == 20220628111752 DropTokenIndexFromCiBuilds: migrated (0.0519s) ==============

marius@roast-carrot ~/W/g/gitlab (339191-remove-plaintext-token-index-from-ci-builds)> VERSION=20220628111752 bin/rails db:migrate:up:main
main: == 20220628111752 DropTokenIndexFromCiBuilds: migrating =======================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- indexes(:ci_builds)
main:    -> 0.0200s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- remove_index(:ci_builds, {:algorithm=>:concurrently, :name=>"index_ci_builds_on_token_partial"})
main:    -> 0.0061s
main: -- execute("RESET statement_timeout")
main:    -> 0.0004s
main: == 20220628111752 DropTokenIndexFromCiBuilds: migrated (0.0359s) ==============

marius@roast-carrot ~/W/g/gitlab (339191-remove-plaintext-token-index-from-ci-builds)> VERSION=20220628111752 bin/rails db:migrate:down:ci
ci: == 20220628111752 DropTokenIndexFromCiBuilds: reverting =======================
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- index_exists?(:ci_builds, :token, {:unique=>true, :where=>"token IS NOT NULL", :name=>"index_ci_builds_on_token_partial", :algorithm=>:concurrently})
ci:    -> 0.0265s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0004s
ci: -- add_index(:ci_builds, :token, {:unique=>true, :where=>"token IS NOT NULL", :name=>"index_ci_builds_on_token_partial", :algorithm=>:concurrently})
ci:    -> 0.0164s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0004s
ci: == 20220628111752 DropTokenIndexFromCiBuilds: reverted (0.0529s) ==============

marius@roast-carrot ~/W/g/gitlab (339191-remove-plaintext-token-index-from-ci-builds)> VERSION=20220628111752 bin/rails db:migrate:down:main
main: == 20220628111752 DropTokenIndexFromCiBuilds: reverting =======================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- index_exists?(:ci_builds, :token, {:unique=>true, :where=>"token IS NOT NULL", :name=>"index_ci_builds_on_token_partial", :algorithm=>:concurrently})
main:    -> 0.0193s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- add_index(:ci_builds, :token, {:unique=>true, :where=>"token IS NOT NULL", :name=>"index_ci_builds_on_token_partial", :algorithm=>:concurrently})
main:    -> 0.0133s
main: -- execute("RESET statement_timeout")
main:    -> 0.0004s
main: == 20220628111752 DropTokenIndexFromCiBuilds: reverted (0.0416s) ==============

Impact

Reduced ci_builds size by 4.4GB

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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