Remove old TODO comment

This TODO comment has been here more than 7 years and doesn't seem to be
a temporary implementation anymore.

railties/lib/rails/generators/rails/app/app_generator.rb

@@ -21,7 +21,6 @@ def #{method}(*args, &block)
         RUBY
       end
 
-      # TODO: Remove once this is fully in place
       def method_missing(meth, *args, &block)
         @generator.send(meth, *args, &block)
       end