Rollup merge of #139908 - Zalathar:no-ascription, r=jieyouxu

parser: Remove old diagnostic notes for type ascription syntax

Type ascription syntax was removed by #109128 in 2023, so “remove this again in a few months” is long overdue.

Happily, this also reduces the amount of parser diagnostic code that cares whether the compiler is unstable.

---

See also the recent #138898, which removed some other related dead code but declined to touch the diagnostics.

It's possible that some of these parser tests are no longer useful at all, but I haven't investigated them for this PR.

Author: matthiaskrgr