gnrc/nib: don't define _set_rtr_adv() twice
gnrc/nib: don't define _set_rtr_adv() twice

It's already defined in `_nib-router.h`, defining it to no-op
in `_nib-6lr.h` makes it impossible to run-time enable advertisements
on non 6lo nodes.

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