Remove the old parser

[lysnikolaou]

Now that beta3 is out and beta4 is scheduled for the end of June, should we maybe start working on removing the old parser? I know @pablogsal already had something in the making, but I guess the review along with things that may need improving will take time.

[pablogsal]

I opened this draft PR with the rebased version of the commit I have started python#20768

[gvanrossum]

I suppose that's reasonable.

[lysnikolaou]

python#20768 is now merged!