Fix incorect error checking

We don't need to check the global stack for errors, as the correct values are already checked by looking at the extruder stacks

cura/Machines/MachineErrorChecker.py

@@ -127,7 +127,7 @@ class MachineErrorChecker(QObject):
 
         # Populate the (stack, key) tuples to check
         self._stacks_and_keys_to_check = deque()
-        for stack in [global_stack] + list(global_stack.extruders.values()):
+        for stack in global_stack.extruders.values():
             for key in stack.getAllKeys():
                 self._stacks_and_keys_to_check.append((stack, key))