sys/fs/constfs: drop dummy implementations
sys/fs/constfs: drop dummy implementations

VFS will handle it if those functions are not implemented.

miri64/RIOT,kYc0o/RIOT,jasonatran/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,ant9000/RIOT,miri64/RIOT,kaspar030/RIOT,miri64/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,ant9000/RIOT,miri64/RIOT,kaspar030/RIOT,kaspar030/RIOT,ant9000/RIOT,ant9000/RIOT,kYc0o/RIOT,jasonatran/RIOT,RIOT-OS/RIOT