Remove es_MX translation

[fitojb]

It is a huge burden for volunteers to try to fully translate a project
this size two times. There is nothing in Git’s terminology that merits
the creation of a duplicate Spanish translation. In Spanish-language
localization of computer software, dialectal files are mostly to address
minor vocabulary differences in colloquial language, meaning that locales
like es_MX or es_ES should only be produced for video games and related
programs, but not projects like Git.

[rimrul]

There are 2 issues with this:

• If we drop one of these translations, we should merge them. There's a bunch of translated strings in documentation.es_MX.po that aren't translated in documentation.es.po. These would be lost if we just drop one of them. We should keep them at least as fuzzy translations. (What weblate calls "Strings marked for editing")
• Even if we drop documentation.es_MX.po from the Github repo, it stays on weblate (as we discovered in Licensing #27/Fix licensing issues #36)