.circleci: remove directory
.circleci: remove directory

This removes building our doc at CircleCI. The doc is now build by the
new Murdock instance.

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