staging: rtl8188eu: remove unused {en,dis}able_interrupt

rtw_hal_enable_interrupt() and rtw_hal_disable_interrupt() are never
referenced, so remove them.

They used to be the only place where the enable_interrupt() and
disable_interrupt() callbacks were called, and the function pointer
is never set, so get rid of the function pointer as well.

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>