Earlier commits impose rules on lifetimes that make generic
destructors safe; thus we no longer need the `#[unsafe_destructor]`
attribute nor its associated check.

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So remove the check for the unsafe_destructor attribute.

And remove outdated compile-fail tests from when lifetime-parameteric
dtors were disallowed/unsafe.

In addition, when one uses the attribute without the associated
feature, report that the attribute is deprecated.

However, I do not think this is a breaking-change, because the
attribute and feature are still currently accepted by the compiler.
(After the next snapshot that has this commit, we can remove the
feature itself and the attribute as well.)

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I consider this to:

Fix #22196

(techincally there is still the post snapshot work of removing the
last remants of the feature and the attribute, but the ticket can
still be closed in my opinion).