AP_HAL_VRBRAIN: remove pointless attach_interrupt override

libraries/AP_HAL_VRBRAIN/GPIO.cpp

@@ -172,14 +172,6 @@ AP_HAL::DigitalSource* VRBRAINGPIO::channel(uint16_t n) {
     return new VRBRAINDigitalSource(0);
 }
 
-/* Interrupt interface: */
-bool VRBRAINGPIO::attach_interrupt(uint8_t interrupt_num,
-                                   irq_handler_fn_t fn,
-                                   INTERRUPT_TRIGGER_TYPE mode)
-{
-    return true;
-}
-
 /*
   return true when USB connected
  */

libraries/AP_HAL_VRBRAIN/GPIO.h

@@ -30,11 +30,6 @@ public:
     /* Alternative interface: */
     AP_HAL::DigitalSource* channel(uint16_t n) override;
 
-    /* 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) override;