stop marking deref_patterns as an incomplete feature

[cyrgani]

This PR removes the incomplete_feature warning for deref_patterns. The reason given for this in the tracking issue (#87121) was

Per policy, the incomplete_feature is supposed to stay on until the feature has an accepted RFC. We're slowly working on writing up that RFC so it'll take some more time unfortunately.

I don't know of any compiler crashes it causes today. The feature should be pretty usable.

However, I could not find any evidence of such a policy. The lint documentation for incomplete_features also only mentions features that are more likely to cause errors.
There are also many other features without an RFC that are not considered incomplete, e.g. macro_metavar_expr_concat, negative_impls or yeet_expr.
The feature does not cause any known ICEs either.

The concrete motivation is to use this feature to replace box_patterns in the compiler and pave the way towards removing that legacy feature.

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

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[jackh726]

This is indeed more or less current policy. This should get an RFC before being marked as incomplete.