result_large_err: get DefId from AdtDef instead of expecting (#15467)

tiny simplification

changelog: none

Author: dswij

clippy_lints/src/functions/result.rs

@@ -97,11 +97,7 @@ fn check_result_unit_err(cx: &LateContext<'_>, err_ty: Ty<'_>, fn_header_span: S
 
 fn check_result_large_err<'tcx>(cx: &LateContext<'tcx>, err_ty: Ty<'tcx>, hir_ty_span: Span, large_err_threshold: u64) {
     if let ty::Adt(adt, subst) = err_ty.kind()
-        && let Some(local_def_id) = err_ty
-            .ty_adt_def()
-            .expect("already checked this is adt")
-            .did()
-            .as_local()
+        && let Some(local_def_id) = adt.did().as_local()
         && let hir::Node::Item(item) = cx.tcx.hir_node_by_def_id(local_def_id)
         && let hir::ItemKind::Enum(_, _, ref def) = item.kind
     {