boards/common/arduino-atmega: remove special case for jiminy-mega256rfr2

The board is deprecated, no need to ignore this board anymore here.

Having the `arduino` and `periph_pwm` features is now required for all
boards using 'arduino-atmega' (as it was except for that board).

If this should change in the future, it should be defined either in each
arduino board, in another board common, or per CPU_MODEL.

boards/common/arduino-atmega/Makefile.features

@@ -7,7 +7,5 @@ FEATURES_PROVIDED += periph_timer
 FEATURES_PROVIDED += periph_uart
 
 # Various other features (if any)
-ifeq (,$(filter jiminy-mega256rfr2,$(BOARD)))
-  FEATURES_PROVIDED += arduino
-  FEATURES_PROVIDED += periph_pwm
-endif
+FEATURES_PROVIDED += arduino
+FEATURES_PROVIDED += periph_pwm