boards/atmega_common: remove weak led_init()
boards/atmega_common: remove weak led_init()

`led_init()` is already protected by an `#ifdef`, so no need for
the weak symbol.

The problem is that the weak function does not get overridden, even when
the board provides it's own led_init(), resulting in no LEDs being
initialized.

jasonatran/RIOT,aeneby/RIOT,aeneby/RIOT,x3ro/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,yogo1212/RIOT,x3ro/RIOT,mtausig/RIOT,yogo1212/RIOT,OlegHahm/RIOT,miri64/RIOT,yogo1212/RIOT,x3ro/RIOT,smlng/RIOT,miri64/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,OlegHahm/RIOT,miri64/RIOT,authmillenon/RIOT,ant9000/RIOT,toonst/RIOT,jasonatran/RIOT,josephnoir/RIOT,miri64/RIOT,kaspar030/RIOT,kaspar030/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,OTAkeys/RIOT,ant9000/RIOT,authmillenon/RIOT,authmillenon/RIOT,OlegHahm/RIOT,aeneby/RIOT,OTAkeys/RIOT,basilfx/RIOT,yogo1212/RIOT,jasonatran/RIOT,ant9000/RIOT,mtausig/RIOT,josephnoir/RIOT,x3ro/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,toonst/RIOT,aeneby/RIOT,kYc0o/RIOT,kYc0o/RIOT,basilfx/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kaspar030/RIOT,OTAkeys/RIOT,yogo1212/RIOT,aeneby/RIOT,x3ro/RIOT,kYc0o/RIOT,mtausig/RIOT,smlng/RIOT,smlng/RIOT,smlng/RIOT,authmillenon/RIOT,OTAkeys/RIOT,basilfx/RIOT,OlegHahm/RIOT,toonst/RIOT,toonst/RIOT,OlegHahm/RIOT,authmillenon/RIOT,ant9000/RIOT,josephnoir/RIOT,kYc0o/RIOT,basilfx/RIOT,basilfx/RIOT,jasonatran/RIOT,kYc0o/RIOT,josephnoir/RIOT,smlng/RIOT,miri64/RIOT,toonst/RIOT,mtausig/RIOT,mtausig/RIOT