Remove ColumnDescriptor structure and usage

[bhufmann]

The ColumnDescriptor data structure (column_descriptor.py) was introduced to support the server-side equivalent data structure which has been serialized in the tree structure by the Trace Compass trace server. However, this was an undocumented and Trace Compass internal data structure. The serialization was removed to align with the TSP specification. After that it's no longer available on the client and cannot be acted on.

In implementation of option --get-tree, the column descriptor is used to print the header information of each column. Since it's not available anymore the header is not printed anymore.

When removing this data structure in the client, the print-out (TreeModel.print()) needs to be fixed as well to print the header information which is provided differently now.