Remove old `#if 0` code from parser
The new parser landed in 0.54, that is, 5 years ago.
I think it's time to remove the plenty of dead #if 0
codes from it.
They could not compile, often they refer to methods and variables (and perhaps concepts) that no longer exist. They pretty much serve documentation purposes only, to see how they used to behave 5 years ago (written in a "language" that we haven't spoken for 5 years).
In the rare case that we still suspect a regression and might want to check how the code used to behave, git history should be good enough for that. Also, in that case it's much more important to figure out the desired behavior (including corner cases) and implement that, rather than compare with an ancient version that might also be buggy or imperfect.
IMHO we should get these out of the way so that we can better focus on the current code. @chpe what do you think?