rtl818x: remove unused conf_erp callback from RF ops struct

The ERP configuration is now done outside RF code in both rtl8180
and rtl8187 drivers.

The conf_erp callback in common RF ops struct is now useless.
Remove it.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>