hir: stop checking codegen fn attrs for constants

See linked comment[1] for context.

1: rust-lang#64809 (comment)

Signed-off-by: David Wood <david@davidtw.co>

src/librustc/hir/check_attr.rs

@@ -112,7 +112,7 @@ impl CheckAttrVisitor<'tcx> {
             return;
         }
 
-        if target == Target::Fn || target == Target::Const {
+        if target == Target::Fn {
             self.tcx.codegen_fn_attrs(self.tcx.hir().local_def_id(item.hir_id));
         }