Turn off physics servers in the Project Manager.

Copying the editor behaviour of deactivating these servers we're not
using, to reduce CPU load.

(cherry picked from commit 27a6ab457b15e448147ab97ab6b23e835ac9e741)

editor/project_manager.cpp

@@ -2374,6 +2374,10 @@ ProjectManager::ProjectManager() {
 		EditorSettings::create();
 	}
 
+	// Turn off some servers we aren't going to be using in the Project Manager.
+	PhysicsServer::get_singleton()->set_active(false);
+	Physics2DServer::get_singleton()->set_active(false);
+
 	EditorSettings::get_singleton()->set_optimize_save(false); //just write settings as they came
 
 	{