COMP: get rid of BuildScripts

I would just check this in but I do think it bears having other eyes on it before checking in.

Hans asked me to get rid of the duplicate ExternalData script.  In a review of the whole build system it appears that the BuildScript directory is no longer relevant.

This does get rid of a script that might be occasionally useful -- runUncrustify.sh, but honestly, that doesn't happen very often, and it was borrowed from ITK, where it can be re-borrowed again. This wasn't actually any part of the build process, and I doubt anyone has bothered to keep it up to date and operational.

The other thing that I have doubts about is the ExternalData_config.cmake.in -- the way that things were set up, the CMake/ExternalData.cmake that refers to it would fail to find it because it wasn't in the same directory.  That is no longer the case.