Remove atexit() call on Win32 code path that cause crashes on mapserv

Reported on http://lists.osgeo.org/pipermail/mapserver-dev/2014-October/014269.html
The cleanup done by the atexit() callback is redundant with the one done
by msCleanup(0) at the end of the main().
Fix crash on CONNECTIONTYPE kerneldensity on Windows (#5019)