Metadata.py: remove AppendXMLNodes() and associated functions

[jjrob]

The AppendXMLNodes() functions were used in MGET 0.x as part of building the ArcGIS .tbx toolbox for ArcGIS Desktop. We no longer build the .tbx toolbox or support ArcGIS Desktop (we build a .atbx toolbox instead, and support ArcGIS Pro). We should remove the AppendXMLNodes() functions, if possible.

[jjrob]

From Metadata.py, we can remove all of these:

• AppendXMLNodes
• AppendPropertyXMLNode
• AppendPropertyDocutilsXMLNodes
• DocutilsXMLCache
• WriteDocutilsXMLCache
• LoadDocutilsXMLCache