qnx4fs: remove remains of the (defunct) write support

commit 945ffe54 ("qnx4: remove write support") removed the (defunct)
write support but missed a chunk of related, dead code.

Signed-off-by: Anders Larsen <al@alarsen.net>
Cc: Jiri Kosina <jkosina@suse.cz>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>