Remove redundant index_events_on_target_type_and_target_id

What does this MR do and why?

The events table has these two indexes:

CREATE INDEX index_events_on_target_type_and_target_id ON events USING btree (target_type, target_id);

CREATE UNIQUE INDEX index_events_on_target_type_and_target_id_and_fingerprint ON events USING btree (target_type, target_id, fingerprint);

The former is made redundant by the latter, so we can remove it.

This will free up ~30G of space.

Screenshots or screen recordings

== 20211104044453 RemoveRedundantEventsIndex: reverting =======================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:events, [:target_type, :target_id], {:name=>:index_events_on_target_type_and_target_id, :algorithm=>:concurrently})
   -> 0.0063s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- add_index(:events, [:target_type, :target_id], {:name=>:index_events_on_target_type_and_target_id, :algorithm=>:concurrently})
   -> 0.0081s
-- execute("RESET statement_timeout")
   -> 0.0007s
== 20211104044453 RemoveRedundantEventsIndex: reverted (0.0180s) ==============

== 20211104044453 RemoveRedundantEventsIndex: migrating =======================
-- transaction_open?()
   -> 0.0000s
-- indexes(:events)
   -> 0.0104s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- remove_index(:events, {:algorithm=>:concurrently, :name=>:index_events_on_target_type_and_target_id})
   -> 0.0049s
-- execute("RESET statement_timeout")
   -> 0.0011s
== 20211104044453 RemoveRedundantEventsIndex: migrated (0.0195s) ==============

How to set up and validate locally

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

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