Remove disable_11b_rates() driver_ops

Remove disable_11b_rates() driver_ops

This function was used unconditionally if wpa_supplicant build
includes CONFIG_P2P=y. Adding a separate driver_ops for such use
is not really useful since the driver wrappers can do the same
internally. Remove this driver_ops and move matching functionality
into driver_nl80211.c which was the only driver wrapper using
this driver_ops callback.

Signed-hostap: Jouni Malinen <j@w1.fi>