b43: Remove unneeded message

The driver can spam the logs with "RX: Packet dropped" messages. These drops
originate from 1. a correpted PLCP, 2. decryption errors, and 3. packet
size underruns. Condition #3 logs a separate message, thus no dropped message
is needed.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>