Cleanup first_mentioned_in_commit_at migration

What does this MR do and why?

This MR cleans up the FixFirstMentionedInCommitAt migration (merged in 14.4) and removes the supporting database index.

Migration

Up:

== 20211207090503 CleanupFirstMentionedInCommitJobs: migrating ================
-- transaction_open?()
   -> 0.0000s
-- indexes(:issue_metrics)
   -> 0.0042s
-- execute("SET statement_timeout TO 0")
   -> 0.0009s
-- remove_index(:issue_metrics, {:algorithm=>:concurrently, :name=>"index_issue_metrics_first_mentioned_in_commit"})
   -> 0.0076s
-- execute("RESET statement_timeout")
   -> 0.0010s

Down:

== 20211207090503 CleanupFirstMentionedInCommitJobs: reverting ================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issue_metrics, :issue_id, {:where=>"EXTRACT(YEAR FROM first_mentioned_in_commit_at) > 2019", :name=>"index_issue_metrics_first_mentioned_in_commit", :algorithm=>:concurrently})
   -> 0.0021s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:issue_metrics, :issue_id, {:where=>"EXTRACT(YEAR FROM first_mentioned_in_commit_at) > 2019", :name=>"index_issue_metrics_first_mentioned_in_commit", :algorithm=>:concurrently})
   -> 0.0072s
-- execute("RESET statement_timeout")
   -> 0.0010s
== 20211207090503 CleanupFirstMentionedInCommitJobs: reverted (0.0209s) =======

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #342532 (closed)