LIBKML: do not delete the libkml singleton factory. Causes weird issues (sometimes...) with Python autotest suite when it explicitely tries to call OGRCleanupAll(). And on Windows there might be cross-heap issues in using delete on the factory pointer (linked to #5775)

git-svn-id: https://svn.osgeo.org/gdal/trunk@28163 f0d54148-0727-0410-94bb-9a71ac55c965