Remove handle_user_key_event from the Sketch

The function is not used anymore, there's a different, better way to
hook into the event handler now.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

KeyboardioFirmware.ino

@@ -10,11 +10,6 @@
 uint8_t primary_keymap = 0;
 uint8_t temporary_keymap = 0;
 
-bool handle_user_key_event(byte row, byte col, uint8_t currentState, uint8_t previousState) {
-  //Serial.print ("user_key_event");
-  return false;
-}
-
 void setup() {
     wdt_disable();
     delay(100);