Remove GetDefId impl for FnRetTy

It was only used in one place, so it seems better to use ordinary
functions.

src/librustdoc/clean/types.rs

@@ -1370,17 +1370,10 @@ crate enum FnRetTy {
     DefaultReturn,
 }
 
-impl GetDefId for FnRetTy {
-    fn def_id(&self) -> Option<DefId> {
-        match *self {
-            Return(ref ty) => ty.def_id(),
-            DefaultReturn => None,
-        }
-    }
-
-    fn def_id_full(&self, cache: &Cache) -> Option<DefId> {
-        match *self {
-            Return(ref ty) => ty.def_id_full(cache),
+impl FnRetTy {
+    crate fn as_return(&self) -> Option<&Type> {
+        match self {
+            Return(ret) => Some(ret),
             DefaultReturn => None,
         }
     }

src/librustdoc/html/render/mod.rs

@@ -1215,7 +1215,7 @@ fn should_render_item(item: &clean::Item, deref_mut_: bool, tcx: TyCtxt<'_>) ->
 fn notable_traits_decl(decl: &clean::FnDecl, cx: &Context<'_>) -> String {
     let mut out = Buffer::html();
 
-    if let Some(did) = decl.output.def_id_full(cx.cache()) {
+    if let Some(did) = decl.output.as_return().and_then(|t| t.def_id_full(cx.cache())) {
         if let Some(impls) = cx.cache().impls.get(&did) {
             for i in impls {
                 let impl_ = i.inner_impl();