Remove unneeded deprecation warning disable comment

`returnNullOnMissingStub` was removed, so we can drop this comment as well.

PiperOrigin-RevId: 576799717

Author: Ilya Yanok

lib/annotations.dart

@@ -102,10 +102,6 @@ class MockSpec<T> {
 
   final Map<Symbol, Function> fallbackGenerators;
 
-  // This is here for the doc comment references to `returnNullOnMissingStub`.
-  // Remove when those are gone.
-  // ignore_for_file: deprecated_member_use_from_same_package
-
   /// Constructs a custom mock specification.
   ///
   /// Specify a custom name with the [as] parameter.