Remove redundant index_epic_issues_on_epic_id

What does this MR do and why?

We have the following two indexes on epic_issues; the former is made redundant by the latter:

CREATE INDEX index_epic_issues_on_epic_id ON epic_issues USING btree (epic_id);

CREATE INDEX index_epic_issues_on_epic_id_and_issue_id ON epic_issues USING btree (epic_id, issue_id);

Migration output

== 20220204053655 RemoveIndexEpicIssuesOnEpicId: migrating ====================
-- transaction_open?()
   -> 0.0000s
-- indexes(:epic_issues)
   -> 0.0061s
-- execute("SET statement_timeout TO 0")
   -> 0.0008s
-- remove_index(:epic_issues, {:algorithm=>:concurrently, :name=>"index_epic_issues_on_epic_id"})
   -> 0.0045s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20220204053655 RemoveIndexEpicIssuesOnEpicId: migrated (0.0142s) ===========

== 20220204053655 RemoveIndexEpicIssuesOnEpicId: reverting ====================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:epic_issues, :epic_id, {:name=>"index_epic_issues_on_epic_id", :algorithm=>:concurrently})
   -> 0.0040s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- add_index(:epic_issues, :epic_id, {:name=>"index_epic_issues_on_epic_id", :algorithm=>:concurrently})
   -> 0.0064s
-- execute("RESET statement_timeout")
   -> 0.0010s
== 20220204053655 RemoveIndexEpicIssuesOnEpicId: reverted (0.0150s) ===========

How to set up and validate locally

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

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