scons/svga: remove opt from the list of valid build types

This reverts commit a5fd54f8.

The whole point was to add a way to pass -DVMX86_STATS to the build,
but we can do that with a command line argument when we invoke scons.

Reviewed-by: José Fonseca <jfonseca@vmware.com>
(cherry picked from commit d6be0b55)