staging: r8188eu: remove unused oid_null_function function

Remove unused oid_null_function function from include/rtw_ioctl.h

Acked-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20210810235047.177883-3-phil@philpotter.co.uk


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>