UBIFS: remove printk

Remove the "UBIFS background thread ubifs_bgd0_0 started" message.
We kill the background thread when we switch to R/O mode, and
start it again whan we switch to R/W mode. OLPC is doing this
many times during boot, and we see this message many times as
well, which is irritating. So just kill the message.

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