Remove VTK-m TestBuild infrastructure
Fixes #309 (closed), and supersedes !1517 (closed)
The purpose of the TestBuild infrastructure was to confirm that VTK-m didn't have any lexical issues when it was a pure header only project. As we now move to have more compiled components the need for this form of testing is mitigated. Combined with the issue of TestBuilds causing MSVC issues, we should just remove this infrastructure.