Remove 13.5 mullvad-browser fluent files from translation

From tpo/translation!118 (comment 3084095).

In !141 (merged) and tpo/translation!118 (merged) we combined the various Fluent files into one, but we left the old fluent files in the translation repository for the 13.5 builds.

Once we are no longer building 13.5 for Mullvad Browser, we should:

1. Stop crowdin from reading or writing any files.
2. Then do a final round of migrations copying the content of each locale's strings into mullvad-browser.ftl.
3. Then delete all the old files.
4. Then point crowdin to the mullvad-browser.ftl file instead.

/cc @ruihildt @emmapeel