Remove invalid language strings_sh.txt file

Serbo-Croatian (sh) language File was added by TranslateWiki back in
2019, but its configuration is incorrect

- The file name does not follow our naming convention (should be
  strings_serbo_croatian.txt)
- Language code `sh` is deprecated according to [1]
- The language is not defined in configuration ($g_language_choices_arr,
  $g_language_auto_map)

It was confirmed in the TranslateWiki support thread [2] that the file
can safely be removed.

Fixes #28860

[1]: https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
[2]: https://translatewiki.net/wiki/Thread:Support/Remove_%27sh%27_(serbo-croatian)_language_%3F