Clean up locales that are not being translated

Ey there:

Testing the fluent file https://gitlab.torproject.org/tpo/applications/torbutton/-/blob/main/chrome/locale/en-US/branding/brand.ftl I realised some issues with the locales.

locale 'ku' is an umbrella for several locales

we have a 'ku' locale directory at https://gitlab.torproject.org/tpo/applications/torbutton/-/blob/main/chrome/locale/ku/

This term means Kurdish, but there are many different Kurdish locales, so this is not really a locale. THe most important locales are Central Kurdish or Sorani (ckb), and Northern Kurdish (kmr), which is also called Kurmanji.

I could not import ku on weblate, the same should happen in transifex, and also it is in English anyway, so we are not losing any translations.

Please remove the 'ku' folders from the repo.

Too many Bengali versions

This may be somehow my responsibility: in Transifex we have the language code 'bn', but for Firefox we need bn-IN or bn-BD. I suggest we stick to one language on the translation platform and if we cannot publish 'bn', we should decide on one country code or publish it in both.