Remove # typed: true sigil

This is the only place in our codebase where sigil was added (rails#53065),
so it was probably added accidentally.

Author: kamipo

activesupport/lib/active_support/testing/notification_assertions.rb

@@ -1,5 +1,3 @@
-
-# typed: true
 # frozen_string_literal: true
 
 module ActiveSupport