UBIFS: remove unneeded call from ubifs_sync_fs

Nowadays VFS always synchronizes all dirty inodes and pages before
calling '->sync_fs()', so remove unneeded 'generic_sync_sb_inodes()'
from 'ubifs_sync_fs()'. It used to be needed, but not any longer.

Pointed-out-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>