staging: rtl8188eu: remove unused rtw_hal_set_def_var

rtw_hal_set_def_var() is never referenced, so remove it.

It used to be the only place where the SetHalDefVarHandler callback was
called, so get rid of the function pointer as well.

Also remove the callback itself, SetHalDefVar8188EUsb(), which was not
called anywhere else.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Greg Kroah-Hartman <gregkh@linux.com>
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>