COMP: Do not package GitPython and PyGithub test directories. Fixes #3749

This commit reverts r23088 and now ensures that the test directories from
the GitPython and PyGithub python module install tree are removed. This
should fix the packaging issue caused by two long paths reported in
issue #3749 when install rules in "SlicerBlockInstallPython.cmake" where
executed.

To conveniently delete folders from the install tree, the CMake function
"ExternalProject_PythonModule_InstallTreeCleanup" has been introduced. Note
that it doesn't yet belong to the Slicer API and exists purely as an
implementation detail.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23465 3bd1e089-480b-0410-8dfb-8563597acbee