Don't special-case ! in stability checks anymore

[GrigorenkoPV]

Two instances of special-casing:

• one for !, added in Warn for #[unstable] on trait impls when it has no effect. #76538 (nearly 6 years ago, Rust v1.48), back when ! was unstable,
• one for fn() -> ! in Allow #[unstable] impls for fn() with unstable abi. #103239 (nearly 4 years ago, Rust v1.66), because -> ! was allowed on stable even with ! being unstable.

Since then, in #103239 (just a week ago, on track to be part of Rust v1.100), ! was stabilized, so the first check became wrong and the second became redundant. Let's remove them.

Fixes #162116.

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[WaffleLapkin]

r=me with green ci

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[mejrs]

I now realize impls for function pointers

#[unstable(feature = "b", issue = "none")]
impl Trait for fn(!) {}

also emit no diagnostic, can you add a test for it?

I think this works correctly but it's untested, can you add one as well?

#[unstable(feature = "b", issue = "none")]
impl Trait for fn(UnstableType) {}

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[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[GrigorenkoPV]

@rustbot ready