Suppress warning: method redefined; discarding old warn_on_pending_cops

This PR suppresses the following warning.

```console
% RUBYOPT=-w bundle exec rspec spec/rubocop/remote_config_spec.rb
(snip)

/Users/koic/src/github.com/rubocop/rubocop/spec/support/suppress_pending_warning.rb:11:
warning: method redefined; discarding old warn_on_pending_cops
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/config_loader.rb:153:
warning: previous definition of warn_on_pending_cops was here
```

cf. rails/rails#23534

spec/support/suppress_pending_warning.rb

@@ -8,6 +8,7 @@
 module RuboCop
   class ConfigLoader
     class << self
+      remove_method :warn_on_pending_cops
       def warn_on_pending_cops(config)
         # noop
       end