QmlDesigner.ItemLibrary: remove forceEntry

This feature allowed items to show up in the library even if the import
was missing. This feature was error prone and increased the complexity
of creating items a lot. Since we now have an import manager this is not
required anymore.

Change-Id: Ia127077be0b6c82a047cbb8f86ac3ab24e2d82ae
Reviewed-by: Marco Bubke <marco.bubke@digia.com>