qnx4fs: small cleanup

Small qnx4 cleanup patch.
- removes .writepage, .write_begin and .write_end (+callback functions)
- removes '.' path checking in namei.c (handled on upper layers)

Signed-off-by: Kai Bankett <chaosman@ontika.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>