Eventual debugging LEDs aren't part of the CPU, but part of the
electronics. Accordingly, define it in config.*.h, not in
arduino_*.h (which would be better named something like
"atmega_*.h).
This meant to be a firmware-provided retract feature but was
never really supported by G-code generators. Without their support
(by issueing M101/M103), it's pretty hard to detect extrusion
pauses, so this feature simply has no future.
As this was on by default, it saves over 200 bytes binary size
in a default configuration.
Markus Hitter