Rollup merge of #142704 - tgross35:remove-concat_idents, r=fee1-dead

Remove the deprecated unstable `concat_idents!` macro

In [rust-lang/rust#137653], the lang and libs-API teams did a joint FCP to deprecate
and eventually remove the long-unstable `concat_idents!` macro. The
deprecation is landing in 1.88, so do the removal here (target version
1.90).

This macro has been superseded by the more recent `${concat(...)}`
metavariable expression language feature, which avoids some of the
limitations of `concat_idents!`. The metavar expression is unstably
available under the [`macro_metavar_expr_concat`] feature.

History is mildly interesting here: `concat_idents!` goes back to 2011
when it was introduced with 513276e595f8 ("Add #concat_idents[] and
#ident_to_str[]"). The syntax looks a bit different but it still works
about the same:

    let asdf_fdsa = "<.<";
    assert(#concat_idents[asd,f_f,dsa] == "<.<");

    assert(#ident_to_str[use_mention_distinction]
           == "use_mention_distinction");

(That test existed from introduction until its removal here.)

Closes: https://github.com/rust-lang/rust/issues/29599

[rust-lang/rust#137653]: https://github.com/rust-lang/rust/pull/137653
[`macro_metavar_expr_concat`]: https://github.com/rust-lang/rust/issues/124225

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