remove references to Property Graph

[jameslamb]

Nightly builds are failing like this:

WARNING: [autosummary] failed to import cugraph.experimental.PropertyGraph.
Possible hints:
* AttributeError: module 'cugraph.experimental' has no attribute 'PropertyGraph'
* ImportError: 
* ModuleNotFoundError: No module named 'cugraph.experimental.PropertyGraph'

Extension error (sphinx.ext.autosummary)!

(build link)

Looks like that's a result of rapidsai/cugraph#5374

This removes all references to cuGraph Property Graphs.

It also updates the description of cugraph-pyg, matching what @alexbarghi-nv has started over in rapidsai/cugraph-gnn#375

[jameslamb]

thanks @BradReesWork !

[jameslamb]

/merge

[acostadon]

Looks good