KWSys: Remove unused CheckCXXSourceRuns cmake module

This file has not been used in KWSys since commit "Moved test for large
file support into kwsysPlatformCxxTests.cxx", 2006-08-25.  CMake 2.6.0
and above come with a copy of this module anyway, and KWSys has required
CMake 2.6.3 since commit "KWSys: Require at least CMake 2.6.3",
2011-03-01.