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 0028860