Don't pre-load engine before other plug-ins

This was a fix for the post-processing plug-in that's no longer required.
It was also only applied when running from source, so obviously it's not required or we'd have seen it in the builds.

cura/CuraApplication.py

@@ -756,7 +756,6 @@ class CuraApplication(QtApplication):
         if not hasattr(sys, "frozen"):
             self._plugin_registry.addPluginLocation(os.path.join(os.path.abspath(os.path.dirname(__file__)), "..", "plugins"))
             self._plugin_registry.loadPlugin("ConsoleLogger")
-            self._plugin_registry.loadPlugin("CuraEngineBackend")
 
         self._plugin_registry.loadPlugins()