UBIFS: remove fast unmounting

This UBIFS feature has never worked properly, and it was a mistake
to add it because we simply have no use-cases. So, lets still accept
the fast_unmount mount option, but ignore it. This does not change
much, because UBIFS commit in sync_fs anyway, and sync_fs is called
while unmounting.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

Documentation/filesystems/ubifs.txt

@@ -79,13 +79,6 @@ Mount options
 
 (*) == default.
 
-norm_unmount (*)	commit on unmount; the journal is committed
-			when the file-system is unmounted so that the
-			next mount does not have to replay the journal
-			and it becomes very fast;
-fast_unmount		do not commit on unmount; this option makes
-			unmount faster, but the next mount slower
-			because of the need to replay the journal.
 bulk_read		read more in one go to take advantage of flash
 			media that read faster sequentially
 no_bulk_read (*)	do not bulk-read