ubifs: Remove obsolete TODO from dfs_file_write()

AFAICT this kind of problems are no longer possible since
debugfs gained file removal protection via
e9117a5a

 ("debugfs: implement per-file removal protection").

Cc: Christoph Hellwig <hch@lst.de>
Cc: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>