TEMP: Revert "Redefine F() as no-op"

This reverts commit d316aa6 to allow running CI tests on a known baseline

Author: facchinm

Diff for: cores/arduino/Arduino.h

@@ -26,9 +26,6 @@
 #include <avr/io.h>
 #include <avr/interrupt.h>
 
-#undef F
-#define F(str) (str)
-
 #ifdef __cplusplus
 extern "C"{
 #endif