Remove code comment related to a RuboCop weird behaviour

See https://gitlab.com/gitlab-org/gitlab/-/merge_requests/83725

spec/fast_spec_helper.rb

 # frozen_string_literal: true
 
-#  $" is $LOADED_FEATURES, but RuboCop didn't like it
 if $".include?(File.expand_path('spec_helper.rb', __dir__))
   # There's no need to load anything here if spec_helper is already loaded
   # because spec_helper is more extensive than fast_spec_helper

spec/spec_helper.rb

 # frozen_string_literal: true
 
-#  $" is $LOADED_FEATURES, but RuboCop didn't like it
 if $".include?(File.expand_path('fast_spec_helper.rb', __dir__))
   warn 'Detected fast_spec_helper is loaded first than spec_helper.'
   warn 'If running test files using both spec_helper and fast_spec_helper,'