examples/minimal: deactivate DEVELHELP
examples/minimal: deactivate DEVELHELP

Having `DEVELHELP` activated for a *minimal* example seems
counter-productive.

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