avr: No need to reset ep0 interrupts on a shutdown

The usb_cdc code will arrange to call usb_read_ep0_setup() after a
shutdown, so no need to have an explicit shutdown handler.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/avr/usbserial.c

@@ -241,10 +241,3 @@ ISR(USB_COM_vect)
     }
     UENUM = old_uenum;
 }
-
-void
-usbserial_shutdown(void)
-{
-    UEIENX = 1<<RXSTPE;
-}
-DECL_SHUTDOWN(usbserial_shutdown);