fuse: delete dead .write_begin and .write_end aops

Ever since 'ea9b990 fuse: implement perform_write', the .write_begin
and .write_end aops have been dead code.

Their task - acquiring a page from the page cache, sending out a write
request and releasing the page again - is now done batch-wise to
maximize the number of pages send per userspace request.

Signed-off-by: Johannes Weiner <jweiner@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>