[ticket/12958] Remove subsilver2 for phpBB 3.2

PHPBB3-12958
[ticket/12958] Remove more references of subsilver2

PHPBB3-12958
Merge remote-tracking branch 'nickvergessen/ticket/12958' into develop

* nickvergessen/ticket/12958:
  [ticket/12958] Remove more references of subsilver2
  [ticket/12958] Remove subsilver2 for phpBB 3.2