qnx4: remove write support

qnx4 wrte support has never been fully implement, is broken since the dawn
of time and hasn't been actively developed since before git history
started.

Instead of letting it further bitrot and complicate API transition (like
the new truncate code) remove it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Anders Larsen <al@alarsen.net>
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>