Mongoid exception messages should not be localized

I understand some developers want localized exception messages, but, I think, most of us don't.
This PR is simply reverting #3169.

Because
- We don't have any official locale files other than English ( https://github.com/mongoid/mongoid/commit/99b04782e086c62e5cf63b09dd627cac134364c1 ).
- It seems @durran intentionally set locale :en. ( #2205 ).
- As a matter of course, this commit does not affect ActiveModel's localized error messages (such as validation error messages).