rustdoc: Fix hiding of private fields

The calculation for whether a field is public or private was tweaked in #13184,
but I forgot to update rustdoc.

Closes #13310

Author: alexcrichton

src/librustdoc/passes.rs

@@ -128,18 +128,12 @@ impl<'a> fold::DocFolder for Stripper<'a> {
                 }
             }
 
-            clean::ViewItemItem(..) => {
+            clean::ViewItemItem(..) | clean::StructFieldItem(..) => {
                 if i.visibility != Some(ast::Public) {
                     return None
                 }
             }
 
-            clean::StructFieldItem(..) => {
-                if i.visibility == Some(ast::Private) {
-                    return None;
-                }
-            }
-
             // handled below
             clean::ModuleItem(..) => {}