Makefile.dep: deduplicate DEFAULT_MODULE
Makefile.dep: deduplicate DEFAULT_MODULE

I think those were added twice by accident.
This cleans that up.

introduced by #13089

RIOT-OS/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,kaspar030/RIOT,authmillenon/RIOT,ant9000/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,yogo1212/RIOT,authmillenon/RIOT,smlng/RIOT,authmillenon/RIOT,OlegHahm/RIOT,ant9000/RIOT,ant9000/RIOT,miri64/RIOT,RIOT-OS/RIOT,miri64/RIOT,OlegHahm/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kYc0o/RIOT,OTAkeys/RIOT,yogo1212/RIOT,jasonatran/RIOT,miri64/RIOT,yogo1212/RIOT,yogo1212/RIOT,smlng/RIOT,smlng/RIOT,yogo1212/RIOT,ant9000/RIOT,basilfx/RIOT,kYc0o/RIOT,OTAkeys/RIOT,smlng/RIOT,miri64/RIOT,OTAkeys/RIOT,kaspar030/RIOT,kYc0o/RIOT,basilfx/RIOT,smlng/RIOT,miri64/RIOT,kaspar030/RIOT,authmillenon/RIOT,ant9000/RIOT,jasonatran/RIOT,jasonatran/RIOT,jasonatran/RIOT,OlegHahm/RIOT,basilfx/RIOT,kYc0o/RIOT,kYc0o/RIOT,jasonatran/RIOT,authmillenon/RIOT,basilfx/RIOT,kaspar030/RIOT,authmillenon/RIOT,basilfx/RIOT,RIOT-OS/RIOT