Fix error with WMS layers not being cleaned fully
Fix error with WMS layers not being cleaned fully

This commit fixes an issue when WMS layers are added and then
removed from the map, they were not deleted from the datastructure
in the MapPanel holding the layers, resulting in an error message from
GeoJs of "Uncaught TypeError: Cannot read property '_exit' of null"
when the DatasetModel was destroyed (deleted).

Kitware/minerva,Kitware/minerva,Kitware/minerva