Remove vega-v5 wrappers

[mattijn]

This PR removes the Vega V5 wrappers from the Altair repository.

It removes them from the generate_schema_wrapper, magics, mimebundle, tests and docs.

Not everything related to vega is removed since the saving utilities requires vega bundles to be included.

[mattijn]

Can you check @joelostblom? I think I removed everything related to the Vega wrapper, but I might have missed some parts in the docs/docstring.

[joelostblom]

Nice, thanks for doing this. Apart from one of the deleted tests I think this looks good. I grepped the repo for "vega" and didn't notice any other places needing an update.

I noticed the name of the branch and just wanted to say that I think it makes sense to have a separate PR for removing the old Vega-Lite versions and limiting this PR to Vega as you have done now 👍

[joelostblom]

I misread the test I thought was accidentally deleted, this looks good.

[joelostblom]

One thing I was thinking here is that maybe we should add an explicit note in the docs that Altair specs can programmatically be turned into VegaLite with to_json, but not Vega specs, and then include the way to create Vega specs directly in Jupyter via IPython.display that you mentioned in #2817?