Remove pending builds covering index

What does this MR do and why?

Related to #332953 (closed)

In %15.2 we've removed the legacy queuing mechanism based on the ci_builds table and now we can remove the index that was supporting it.

There is not usage data for this index in the last 8 weeks.

Because of gitlab-com/gl-infra/production#6821 (closed) we need to drop the index in production via a Change Request before merging this MR: gitlab-com/gl-infra/production#7477 (closed)

Screenshots or screen recordings

> bin/rails db:migrate
main: == 20220720090354 RemovePendingBuildsCoveringIndexFromCiBuilds: migrating =====
main: -- transaction_open?()
main:    -> 0.0000s
main: -- indexes(:ci_builds)
main:    -> 0.0207s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0004s
main: -- remove_index(:ci_builds, {:algorithm=>:concurrently, :name=>"index_ci_builds_runner_id_pending_covering"})
main:    -> 0.0032s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20220720090354 RemovePendingBuildsCoveringIndexFromCiBuilds: migrated (0.0341s)

ci: == 20220720090354 RemovePendingBuildsCoveringIndexFromCiBuilds: migrating =====
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- indexes(:ci_builds)
ci:    -> 0.0191s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0005s
ci: -- remove_index(:ci_builds, {:algorithm=>:concurrently, :name=>"index_ci_builds_runner_id_pending_covering"})
ci:    -> 0.0037s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0004s
ci: == 20220720090354 RemovePendingBuildsCoveringIndexFromCiBuilds: migrated (0.0277s)

> bin/rails db:rollback:main
main: == 20220720090354 RemovePendingBuildsCoveringIndexFromCiBuilds: reverting =====
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0004s
main: -- indexes(:ci_builds)
main:    -> 0.0184s
main: -- current_schema()
main:    -> 0.0002s
main: -- execute("CREATE INDEX CONCURRENTLY index_ci_builds_runner_id_pending_covering ON ci_builds (runner_id, id) INCLUDE (project_id) WHERE status = 'pending' AND type = 'Ci::Build'")
main:    -> 0.0057s
main: -- execute("RESET statement_timeout")
main:    -> 0.0004s
main: == 20220720090354 RemovePendingBuildsCoveringIndexFromCiBuilds: reverted (0.0293s)

> bin/rails db:rollback:ci
ci: == 20220720090354 RemovePendingBuildsCoveringIndexFromCiBuilds: reverting =====
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0004s
ci: -- indexes(:ci_builds)
ci:    -> 0.0187s
ci: -- current_schema()
ci:    -> 0.0004s
ci: -- execute("CREATE INDEX CONCURRENTLY index_ci_builds_runner_id_pending_covering ON ci_builds (runner_id, id) INCLUDE (project_id) WHERE status = 'pending' AND type = 'Ci::Build'")
ci:    -> 0.0061s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0003s
ci: == 20220720090354 RemovePendingBuildsCoveringIndexFromCiBuilds: reverted (0.0302s)

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