AP_HAL_Empty: remove pointless attach_interrupt override

libraries/AP_HAL_Empty/GPIO.cpp

@@ -27,13 +27,6 @@ AP_HAL::DigitalSource* GPIO::channel(uint16_t n) {
     return new DigitalSource(0);
 }
 
-/* Interrupt interface: */
-bool GPIO::attach_interrupt(uint8_t interrupt_num,
-                            irq_handler_fn_t fn,
-                            INTERRUPT_TRIGGER_TYPE mode) {
-    return true;
-}
-
 bool GPIO::usb_connected(void)
 {
     return false;

libraries/AP_HAL_Empty/GPIO.h

@@ -14,11 +14,6 @@ public:
     /* Alternative interface: */
     AP_HAL::DigitalSource* channel(uint16_t n);
 
-    /* Interrupt interface: */
-    bool    attach_interrupt(uint8_t interrupt_num,
-                             irq_handler_fn_t fn,
-                             INTERRUPT_TRIGGER_TYPE mode) override;
-
     /* return true if USB cable is connected */
     bool    usb_connected(void);
 };