QmlDesigner: Clean up form editor code

Change-Id: Ife16e84a7b9d1c41ec6481c7d91a1872281a1064
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner: Rename removeVariantProperty in removeProperty

Change-Id: Id350f7d3e215d08210a3a535f89b12fe15928321
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>