Remove feature(string_deref_patterns)

[Zalathar]

The older string_deref_patterns feature has been superseded by the newer and more general deref_patterns feature. Removing string-deref-patterns allows us to get rid of a few tricky special cases in match lowering, which are different from the special cases used by deref-patterns.

The handful of existing tests for string_deref_patterns have been migrated to use deref_patterns instead. Current nightly users of the older feature should hopefully be able to migrate to the newer feature without too much trouble.

Note that deref_patterns is currently marked as an “incomplete” feature, because it doesn't have an accepted RFC. But string_deref_patterns doesn't appear to have ever had an accepted RFC either, so arguably it should have been marked incomplete too.

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• Tracking issue for both features: Tracking issue: deref patterns #87121
• Original implementation: Minimal implementation of implicit deref patterns for Strings #98914
• Zulip thread: Can we remove #![feature(string_deref_patterns)]?