Remove i18n backport tool at the moment because of translation format changed (#36643)

Starting with v1.26, Gitea uses a JSON configuration file format instead
of the INI format used in v1.25 and earlier versions.

Because of this fundamental format change, a clean translation backport
to the v1.25 branch (or earlier release branches) is not feasible.The
recommended approach is:
- Wait until the release/v1.26 branch is created after the official
v1.26 release.
- Then introduce a new JSON-based configuration (or
migration/compatibility layer) on top of that branch.

Author: lunny

CONTRIBUTING.md

@@ -183,7 +183,7 @@ Here's how to run the test suite:
 ## Translation
 
 All translation work happens on [Crowdin](https://translate.gitea.com).
-The only translation that is maintained in this repository is [the English translation](https://github.com/go-gitea/gitea/blob/main/options/locale/locale_en-US.ini).
+The only translation that is maintained in this repository is [the English translation](https://github.com/go-gitea/gitea/blob/main/options/locale/locale_en-US.json).
 It is synced regularly with Crowdin. \
 Other locales on main branch **should not** be updated manually as they will be overwritten with each sync. \
 Once a language has reached a **satisfactory percentage** of translated keys (~25%), it will be synced back into this repo and included in the next released version.