Remove unneeded index

- add changelog

changelogs/unreleased/297503-cablett-remove-index.yml

+---
+title: Remove unneeded group label index
+merge_request: 51676
+author:
+type: other

db/migrate/20210114033715_remove_group_id_title_index.rb

+# frozen_string_literal: true
+
+class RemoveGroupIdTitleIndex < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'index_labels_on_group_id_and_title_with_null_project_id'
+  LABELS_TABLE = :labels
+
+  def up
+    remove_concurrent_index_by_name LABELS_TABLE, INDEX_NAME
+  end
+
+  def down
+    add_concurrent_index LABELS_TABLE, [:group_id, :title], where: 'project_id IS NULL', name: INDEX_NAME
+  end
+end

db/schema_migrations/20210114033715

+69aae8d967fdb8af816a969fd818ed325b8d780b4faaa205c78a66c5d533ab2a
\ No newline at end of file

db/structure.sql

@@ -22045,8 +22045,6 @@ CREATE UNIQUE INDEX index_labels_on_group_id_and_project_id_and_title ON labels
 
 CREATE UNIQUE INDEX index_labels_on_group_id_and_title_unique ON labels USING btree (group_id, title) WHERE (project_id IS NULL);
 
-CREATE INDEX index_labels_on_group_id_and_title_with_null_project_id ON labels USING btree (group_id, title) WHERE (project_id IS NULL);
-
 CREATE INDEX index_labels_on_project_id ON labels USING btree (project_id);
 
 CREATE UNIQUE INDEX index_labels_on_project_id_and_title_unique ON labels USING btree (project_id, title) WHERE (group_id IS NULL);