ufs: remove unused fs64_add and fs64_sub

remove fs64_add and fs64_sub - they probably weren't ever used because
their prototypes used u32 instead of __fs64

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Evgeniy Dushistov <dushistov@mail.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>