boards/slwstk6220a: remove useless provided gpio features
boards/slwstk6220a: remove useless provided gpio features

These are provided at CPU level.

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