Update python/dask cugraph implementation to use new PLC features from #3947

[rlratzel]

When #3947 is closed, the cugraph python code for MG graphs needs to be updated.
There are three updates here, and I can create separate issues if that's better.

Remove code that computes num_edges from a dask_cudf edgelist

Once #3947 is closed, the num_edges arg to the PLC MGGraph is optional and no longer needs to be computed. This will allow the python code to bypass the need to persist/compute the distributed edgelist.

Remove code that combines (and modifies) the copied input dask_cudf edgelist

Once #3947 is closed, the code that combines the src/dst columns from each partition can be removed. This should greatly simplify the MG graph creation code.

Re-enable MG tests in CI

Once the above changes are made, MG tests in CI should pass reliably.

[BradReesWork]

closed by PR #3941