boost-graph: Remove blueprint
boost-graph is a header-only library (unless one needs the GraphViz input parser or the GraphML parser). digikam and codevis (soon) only need the headers. The headers are provided by boost-headers.
Note that digikam doesn't run any craft builds so that a test build doesn't make much sense. I checked that digikam simply does find_package(Boost)
which is the correct way to look for the Boost headers.