Avoid destroying yet alive color devices

This is a typo that I forgot to fix, therefore not requesting code
review. After QVector::erase() has been called, we cannot use the
iterator as it will lead to undefined behavior.

colormanager.cpp

@@ -70,7 +70,7 @@ void ColorManager::handleOutputDisabled(AbstractOutput *output)
     ColorDevice *device = *it;
     d->devices.erase(it);
     emit deviceRemoved(device);
-    delete *it;
+    delete device;
 }
 
 } // namespace KWin