Doc: Fixed "QML Types" node in Assistant

Removed "class" from the "selectors" variable in qdocconf file.

Task-number: QTBUG-29242
Change-Id: I960dcbdb22358e033177b0ccb9c211136c0e4dcb
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Geir Vattekar <geir.vattekar@digia.com>