[es-mx] Remove Spanish (Mexico) translation

This translation was copied verbatim from en in 02f9838 (as the
398db5e blob, which was added by b26a581).  All subsequent changes
have been tree-wide, so we don't lose any information by removing the
translation.  Future translators should seed their efforts with an
up-to-date branch from another language.
Merge pull request #371 from wking/remove-es-mx

[es-mx] Remove Spanish (Mexico) translation