staging: rtl8723bs: remove mgmt_frame_register method

This was changed in cfg80211, so having it broke things, but
there's no need to adjust since it's an empty implementation.
Just remove it.

Fixes: 6cd536fe ("cfg80211: change internal management frame registration API")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20200428101400.ae19d651ec38.Ieb15844bb5ab93b3d7931d6561f42e3316ef8251@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>