COMP: Remove custom version of FindSubversion.cmake not needed for CMake >= 2.8.3

In commit r16389, a custom version of the find module has been added
so that the associated macro can return the root url of the repository.

Starting with CMake 2.8.3 (commit 80bad29), the CMake built-in macro has
been updated to return the root.

Let's also note that the module provided with CMake >= 2.8.3 also include
a fix allowing the macro "Subversion_WC_INFO" to run as expected on non-US
system. ASsociated CMake issue is http://www.cmake.org/Bug/view.php?id=11273

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21866 3bd1e089-480b-0410-8dfb-8563597acbee