Undo the redefinition of FPSTR from 8266 merge (espressif#2726)

Fixes Arduino.h redefinition errors.

Author: earlephilhower

Diff for: cores/esp32/WString.h

@@ -35,8 +35,7 @@ class StringSumHelper;
 // an abstract class used as a means to proide a unique pointer type
 // but really has no body
 class __FlashStringHelper;
-#define FPSTR(pstr_pointer) (reinterpret_cast<const __FlashStringHelper *>(pstr_pointer))
-#define F(string_literal) (FPSTR(PSTR(string_literal)))
+#define F(string_literal) (reinterpret_cast<const __FlashStringHelper *>(PSTR(string_literal)))
 
 // The string class
 class String {