main: Remove serialEvent no-op functions

fixed: https://github.com/ClangBuiltArduino/core_arduino-avr/commit/72ac6890343ec86e8b25369b1dcfd338a9e56e90

Author: beakthoven

main.cpp

@@ -1,19 +1,5 @@
 #include <Arduino.h>
 
-void serialEvent() {
-}
-
-void serialEvent1() {
-	serialEvent();
-}
-void serialEvent2() {
-	serialEvent();
-}
-void serialEvent3() {
-	serialEvent();
-}
-
-
 void setup() {
 	Serial.begin(9600);
 	Serial.print("started\n");