Remove redundant index on taggings table

Remove index `index_taggings_on_taggable_id_and_taggable_type`. The
index `index_taggings_on_taggable_id_and_taggable_type_and_context`
already has the same leading columns.

Changelog: other

db/post_migrate/20210923133143_remove_redundant_taggings_index.rb

+# frozen_string_literal: true
+
+class RemoveRedundantTaggingsIndex < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  INDEX_NAME = :index_taggings_on_taggable_id_and_taggable_type
+
+  def up
+    remove_concurrent_index_by_name :taggings, INDEX_NAME
+  end
+
+  def down
+    add_concurrent_index :taggings, [:taggable_id, :taggable_type], name: INDEX_NAME
+  end
+end

db/schema_migrations/20210923133143

+d2736a06009d6232d832a03d6842a81b1de2ce79b901331a0e09ac40fc51a463
\ No newline at end of file

db/structure.sql

@@ -26510,8 +26510,6 @@ CREATE UNIQUE INDEX index_system_note_metadata_on_note_id ON system_note_metadat
 
 CREATE INDEX index_taggings_on_tag_id ON taggings USING btree (tag_id);
 
-CREATE INDEX index_taggings_on_taggable_id_and_taggable_type ON taggings USING btree (taggable_id, taggable_type);
-
 CREATE INDEX index_taggings_on_taggable_id_and_taggable_type_and_context ON taggings USING btree (taggable_id, taggable_type, context);
 
 CREATE UNIQUE INDEX index_tags_on_name ON tags USING btree (name);