Drop unused mirror_data index

What does this MR do?

This covers the follow-up #326009 (closed)

After more index_mirror_data_non_scheduled_or_started was introduced,
the old index index_mirror_data_on_next_execution_and_retry_count became obsolete.

We could perform one more check after April 5th on the production, to make sure it's not used.

rake db:migrate

== 20210401134455 RemoveIndexMirrorDataOnNextExecutionAndRetryCount: migrating
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:project_mirror_data, [:next_execution_timestamp, :retry_count], {:name=>"index_mirror_data_on_next_execution_and_retry_count", :algorithm=>:concurrently})
   -> 0.0035s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:project_mirror_data, {:name=>"index_mirror_data_on_next_execution_and_retry_count", :algorithm=>:concurrently, :column=>[:next_execution_timestamp, :retry_count]})
   -> 0.0095s
-- execute("RESET ALL")
   -> 0.0005s
== 20210401134455 RemoveIndexMirrorDataOnNextExecutionAndRetryCount: migrated (0.0148s)

rake db:rollback

== 20210401134455 RemoveIndexMirrorDataOnNextExecutionAndRetryCount: reverting
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:project_mirror_data, [:next_execution_timestamp, :retry_count], {:name=>"index_mirror_data_on_next_execution_and_retry_count", :algorithm=>:concurrently})
   -> 0.0036s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:project_mirror_data, [:next_execution_timestamp, :retry_count], {:name=>"index_mirror_data_on_next_execution_and_retry_count", :algorithm=>:concurrently})
   -> 0.0056s
-- execute("RESET ALL")
   -> 0.0005s
== 20210401134455 RemoveIndexMirrorDataOnNextExecutionAndRetryCount: reverted (0.0112s)

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

I tested the migration locally

Adam checked that is not used on prod: #326009 (comment 542204857)

Security

N/A

Related to #326009 (closed)