sys/arduino: drop cpp feature dependency

Currently, the arduino module depends on the "cpp" feature. As AVR's
don't have libstdc++, they don't provide that feature, as otherwise all
cpp examples would break.  But the AVR does provide a cpp compiler and
thus the arduino module compiles just fine.

This comit removes the cpp dependency from arduino module.

Makefile.dep

@@ -534,7 +534,6 @@ endif
 
 ifneq (,$(filter arduino,$(USEMODULE)))
   FEATURES_REQUIRED += arduino
-  FEATURES_REQUIRED += cpp
   USEMODULE += xtimer
 endif