Remove redundant index on epics

🔍 What does this MR do?

The epics table has the following two indexes:

"index_epics_on_group_id_and_iid" UNIQUE, btree (group_id, iid)
"index_epics_on_group_id" btree (group_id)

index_epics_on_group_id is made redundant by the first index.

This MR removes index_epics_on_group_id, freeing up some space.

🐘 Database

→ be rake db:migrate:redo
== 20210415144538 RemoveIndexEpicsOnGroupIdFromEpics: reverting ===============
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:epics, :group_id, {:name=>"index_epics_on_group_id", :algorithm=>:concurrently})
   -> 0.0135s
-- execute("SET statement_timeout TO 0")
   -> 0.0014s
-- add_index(:epics, :group_id, {:name=>"index_epics_on_group_id", :algorithm=>:concurrently})
   -> 0.0130s
-- execute("RESET ALL")
   -> 0.0016s
== 20210415144538 RemoveIndexEpicsOnGroupIdFromEpics: reverted (0.0315s) ======

== 20210415144538 RemoveIndexEpicsOnGroupIdFromEpics: migrating ===============
-- transaction_open?()
   -> 0.0000s
-- indexes(:epics)
   -> 0.0123s
-- execute("SET statement_timeout TO 0")
   -> 0.0010s
-- remove_index(:epics, {:algorithm=>:concurrently, :name=>"index_epics_on_group_id"})
   -> 0.0057s
-- execute("RESET ALL")
   -> 0.0019s
== 20210415144538 RemoveIndexEpicsOnGroupIdFromEpics: migrated (0.0226s) ======

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