Don't make rustc_insignificant_dtor feature gate

This isn't a feature gate, it's an attribute that is feature gated
behind the `rustc_attrs` attribute. Closes #85680.

compiler/rustc_feature/src/active.rs

@@ -692,7 +692,6 @@ pub const INCOMPLETE_FEATURES: &[Symbol] = &[
     sym::capture_disjoint_fields,
     sym::inherent_associated_types,
     sym::type_alias_impl_trait,
-    sym::rustc_insignificant_dtor,
     sym::unnamed_fields,
 ];