Remove duplicate definition of `show_config_variable` macro

In 5ed507cf43e048f0a6a527c97ea6ed2e8cdb451f as part of #98,
[`show_config_variable` was duplicated in Arduino.mk file](https://github.com/sudar/Arduino-Makefile/pull/98#commitcomment-3561542)

But it was later found that `ARDMK_DIR` is still not getting printed
(issue #111). This commit therefore removes the duplicated macro
definition.