gnrc_sixlowpan_nd: remove unused config variable (and its header)
gnrc_sixlowpan_nd: remove unused config variable (and its header)

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