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

This reverts commit a5fd54f8bf6713312fa5efd7ef5cd125557a0ffe.

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>
diff --git a/common.py b/common.py
index be3ccfc..e2afff9 100644
--- a/common.py
+++ b/common.py
@@ -86,7 +86,7 @@
         from SCons.Options.EnumOption import EnumOption
     opts.Add(EnumOption('build', 'build type', 'debug',
                         allowed_values=('debug', 'checked', 'profile',
-                                        'release', 'opt')))
+                                        'release')))
     opts.Add(BoolOption('verbose', 'verbose output', 'no'))
     opts.Add(EnumOption('machine', 'use machine-specific assembly code',
                         default_machine,