[ticket/8700] Remove unused language strings

Most of the strings were obsoleted by the removal of:
* Imagesets
* Themes
* Templates
* Editor
* Exporter

PHPBB3-8700
[ticket/8700] Use L_COLON instead of adding ":" manually

PHPBB3-8700
Merge remote-tracking branch 'nickvergessen/ticket/8700' into develop-ascraeus

* nickvergessen/ticket/8700:
  [ticket/8700] Use L_COLON instead of adding ":" manually
  [ticket/8700] Remove unused language strings