remove setupUSB weak hook and replace with global constructors

thanks @matthijskooijman

Author: facchinm

hardware/arduino/avr/cores/arduino/main.cpp

@@ -37,7 +37,6 @@ int main(void)
 	initVariant();
 
 #if defined(USBCON)
-	setupUSB();
 	USBDevice.attach();
 #endif