Fix a crash on exit. The SGEnviro destructor was attempting to delete an

object that had already been deleted by the sound manager.  Since SGEnviro
doesn't own this object, I removed the delete from it's destructor.

simgear/environment/visual_enviro.cxx

@@ -187,7 +187,7 @@ SGEnviro::SGEnviro() :
 }
 
 SGEnviro::~SGEnviro(void) {
-	if (sampleGroup) delete sampleGroup;
+  // if (sampleGroup) delete sampleGroup;
 
   // OSGFIXME
   return;