Delete From impls of ast enums

[dtolnay]

See #1652.

I have not found these impls valuable in practice.

They are originally from #144.

[workingjubilee]

This removal breaks multiple common ecosystem crates, including some that rustc depends on.

[workingjubilee]

I suppose, given that this would have to wait for syn 3, that maybe enough crates will have moved on to syn 2 that they can begin migrating to this new syn and overcome the additional points of friction. Though, it is tempting to report a bug if merely expanding and compiling a few From impls is that much of rustc's compile time of syn...

Or larger projects can embrace the tragicomedy of there being 3 syns in their repo.

The possibilities abound!