Remove unused index index_for_resource_group on ci_builds

A new index was recently added to cover this use case.

Changelog: performance

db/migrate/20211108211434_remove_index_for_resource_group.rb

+# frozen_string_literal: true
+
+class RemoveIndexForResourceGroup < Gitlab::Database::Migration[1.0]
+  INDEX_NAME = 'index_for_resource_group'
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index_by_name :ci_builds, INDEX_NAME
+  end
+
+  def down
+    # rubocop:disable Migration/PreventIndexCreation
+    add_concurrent_index :ci_builds, [:resource_group_id, :id], where: 'resource_group_id IS NOT NULL', name: INDEX_NAME
+    # rubocop:enable Migration/PreventIndexCreation
+  end
+end

db/schema_migrations/20211108211434

+06cbecc52e62a48664ae486181047d8ca4d71a7991ba36bdca4bfa44257627f3
\ No newline at end of file

db/structure.sql

@@ -25893,8 +25893,6 @@ CREATE UNIQUE INDEX index_feature_gates_on_feature_key_and_key_and_value ON feat
 
 CREATE UNIQUE INDEX index_features_on_key ON features USING btree (key);
 
-CREATE INDEX index_for_resource_group ON ci_builds USING btree (resource_group_id, id) WHERE (resource_group_id IS NOT NULL);
-
 CREATE INDEX index_for_status_per_branch_per_project ON merge_trains USING btree (target_project_id, target_branch, status);
 
 CREATE INDEX index_fork_network_members_on_fork_network_id ON fork_network_members USING btree (fork_network_id);