rt2x00: Remove SKBDESC_L2_PADDED flag.

With the improved L2 padding code, this flag is no longer necessary, as the
rt2x00queue_remove_l2pad is capable of detecting by itself if L2 padding is
applied.
For received frames the RX descriptor flag is still being checked.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <ivdoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>