Suppress Ruby's warning

This PR suppresses the following Ruby's warning.

```console
% cd path/to/repo/rubocop-rails
% bundle exec rake
(snip)

/Users/koic/src/github.com/rubocop-hq/rubocop-rails/lib/rubocop/cop/rails/uniq_before_pluck.rb:71:
warning: too many arguments for format string
```

Author: koic

lib/rubocop/cop/rails/uniq_before_pluck.rb

@@ -67,8 +67,7 @@ def on_send(node)
 
           return unless method
 
-          add_offense(node, location: :selector,
-                            message: format(MSG, method: method))
+          add_offense(node, location: :selector)
         end
 
         def autocorrect(node)