Drop index_ci_builds_on_protected index

Ref: #333814 (closed)

What does this MR do?

This MR removes index_ci_builds_on_protected index which is no longer needed.

Database review

Migration output

$ bin/rails db:migrate
== 20210616154808 RemoveCiBuildProtectedIndex: migrating ======================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:ci_builds, :protected, {:name=>"index_ci_builds_on_protected", :algorithm=>:concurrently})
   -> 0.0118s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:ci_builds, {:name=>"index_ci_builds_on_protected", :algorithm=>:concurrently, :column=>:protected})
   -> 0.0136s
-- execute("RESET ALL")
   -> 0.0006s
== 20210616154808 RemoveCiBuildProtectedIndex: migrated (0.0273s) =============

Migration rollback

$ bin/rails db:rollback
== 20210616154808 RemoveCiBuildProtectedIndex: reverting ======================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:ci_builds, :protected, {:name=>"index_ci_builds_on_protected", :algorithm=>:concurrently})
   -> 0.0091s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:ci_builds, :protected, {:name=>"index_ci_builds_on_protected", :algorithm=>:concurrently})
   -> 0.0096s
-- execute("RESET ALL")
   -> 0.0005s
== 20210616154808 RemoveCiBuildProtectedIndex: reverted (0.0205s) =============

Does this MR meet the acceptance criteria?

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• This change is backwards compatible across updates, or this does not apply.

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