Drop index_ci_builds_on_protected index

This commit removes index_ci_builds_on_protected index which is
no longer needed.

Changelog: removed

db/migrate/20210616154808_remove_ci_build_protected_index.rb

+# frozen_string_literal: true
+
+class RemoveCiBuildProtectedIndex < ActiveRecord::Migration[6.1]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  INDEX_NAME = 'index_ci_builds_on_protected'
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index :ci_builds, :protected, name: INDEX_NAME
+  end
+
+  def down
+    add_concurrent_index :ci_builds, :protected, name: INDEX_NAME
+  end
+end

db/schema_migrations/20210616154808

+adfa07888317cb7f22062f5d23906bc01de579305acb3bb3384f18e3f581a87b
\ No newline at end of file

db/structure.sql

@@ -22816,8 +22816,6 @@ CREATE INDEX index_ci_builds_on_project_id_and_name_and_ref ON ci_builds USING b
 
 CREATE INDEX index_ci_builds_on_project_id_for_successfull_pages_deploy ON ci_builds USING btree (project_id) WHERE (((type)::text = 'GenericCommitStatus'::text) AND ((stage)::text = 'deploy'::text) AND ((name)::text = 'pages:deploy'::text) AND ((status)::text = 'success'::text));
 
-CREATE INDEX index_ci_builds_on_protected ON ci_builds USING btree (protected);
-
 CREATE INDEX index_ci_builds_on_queued_at ON ci_builds USING btree (queued_at);
 
 CREATE INDEX index_ci_builds_on_runner_id_and_id_desc ON ci_builds USING btree (runner_id, id DESC);