Remove temporary blocking issues indexes

What does this MR do?

Remove temporary indexes used to schedule background migrations on !42277 (merged)

Migration output

== 20201118181411 RemoveTemporaryBlockingIssuesIndex: migrating ===============
-- transaction_open?()
   -> 0.0000s
-- indexes(:issue_links)
   -> 0.0079s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- remove_index(:issue_links, {:algorithm=>:concurrently, :name=>"tmp_idx_blocking_type_links"})
   -> 0.0098s
-- execute("RESET ALL")
   -> 0.0018s
-- transaction_open?()
   -> 0.0000s
-- indexes(:issue_links)
   -> 0.0043s
-- remove_index(:issue_links, {:algorithm=>:concurrently, :name=>"tmp_idx_blocked_by_type_links"})
   -> 0.0052s
-- transaction_open?()
   -> 0.0000s
-- indexes(:issues)
   -> 0.0133s
-- remove_index(:issues, {:algorithm=>:concurrently, :name=>"tmp_idx_index_issues_with_outdate_blocking_count"})
   -> 0.0085s
== 20201118181411 RemoveTemporaryBlockingIssuesIndex: migrated (0.0577s) ======

related to #258973 (closed)