staging: rtl8188eu: Remove function rtw_getbbrfreg_cmdrsp_callback()

rtw_getbbrfreg_cmdrsp_callback() is a callback for _Read_BBREG and _Read_RFREG
commands but these commands are never issued by driver.

Signed-off-by: navin patidar <navin.patidar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>