Remove unused environment variables and related functions

[rikosellic]

It seems we only need CARGO_VERUS, VERUSFMT, and VERUSDOC now.

[rikosellic]

Fix #30.

[Marsman1996]

Well, actually I think we still need get_rustdoc.

Curretly in cargo dv doc, we just

rust-deductive-verifier/src/doc.rs

Line 85 in d2061d2

let mut cmd = Command::new("rustdoc");

And it should be

let rustdoc = verus::get_rustdoc();

So it could report missing error when rustdoc is not found.

[rikosellic]

Fixed. Also add a bin_name function to deal with Windows .exe uniformly.

[Marsman1996]

LGTM
And make doc works in Linux and macOS