Remove translation from constant in `app/models/merge_request.rb`

The following discussion from !21034 (merged) should be addressed:

• @engwan started a discussion: (+1 comment)

  I think translations won't work here because we're saving it in a class constant and it will always return this message in the default locale?

  I guess we don't really need these to be translated? Are these exceptions caught and shown to the user?

You are right. We don't need these to be translated. They are not shown to the user. I have removed translation. Please note that we have something similar in merge_request.rb