boards/native: remove now unnecessary `-g3` CFLAGS
boards/native: remove now unnecessary `-g3` CFLAGS

dbd865413c603294eebc2fad79ecf60078dfeb97 made `-g3` default for
`make all` so all targets dependent on `make all` do not require it any
longer.

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