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,
 ];
 

src/test/ui/feature-gates/rustc_insignificant_dtor.rs

@@ -0,0 +1,2 @@
+#![feature(rustc_insignificant_dtor)] //~ ERROR unknown feature `rustc_insignificant_dtor`
+fn main() {}

src/test/ui/feature-gates/rustc_insignificant_dtor.stderr

@@ -0,0 +1,9 @@
+error[E0635]: unknown feature `rustc_insignificant_dtor`
+  --> $DIR/rustc_insignificant_dtor.rs:1:12
+   |
+LL | #![feature(rustc_insignificant_dtor)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0635`.