Make unregister_binfmt() return void

list_del() hardly can fail, so checking for return value is pointless
(and current code always return 0).

Nobody really cared that return value anyway.

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>