Do not close the engine from our side when finished with slicing

The engine will close itself when done. Closing it from our side
actually introduces errors as things can still be busy processing

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -213,13 +213,6 @@ class CuraEngineBackend(Backend):
             self._message.hide()
             self._message = None
 
-        if self._always_restart:
-            try:
-                self._process.terminate()
-                self._createSocket()
-            except: # terminating a process that is already terminating causes an exception, silently ignore this.
-                pass
-
     def _onGCodeLayerMessage(self, message):
         self._scene.gcode_list.append(message.data.decode("utf-8", "replace"))