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>