#35 virtual layer links not deleted by mapping delete

[Joseph Coffland]

Deleting a mapping deleted the related virtual layer
nodes so that the virtual layer will correctly reflect
mapping layer changes. However, the links to the
deleted virtual nodes are not removed. Visually they
dissapear but the remain in the YML. This can cause
the YMLEditor to emit head or tail not found warnings.
This warning can be safely ignored.

No workaround is necessary.