Remove obsolete FindGit.cmake

Now that we require CMake 2.8.8, we don't need to package FindGit.cmake,
which is available since 2.8.2.  From 2.8.8 onwards, FindGit.cmake also
provides a version string directly, so we don't need to get it
ourselves.

Change-Id: Id5739af8d8e026a87a478b669b5c9c23adb28aee