parser: Remove old diagnostic notes for type ascription syntax #139908
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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.