staging: rtl8712: remove unused function

mp_query_drv_var() was only being used by
oid_rt_pro_query_dr_variable_hdl() but after commit
<f4f5a59f> mp_query_drv_var() became
unused. so it is safe to remove it.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>