[cleanup] Remove internal vs external distinction

The internal vs external build distinction is no longer useful: it was created when DR was used primarily as a commercial product. Today we build release-external in Travis but only test the release-internal build in CDash and occasionally it fails and takes time to fix. This issue covers examining the details and trying to get rid of the different builds: possibly by making internal==debug and external==release, though not if that prevents changing key options in release build.