Remove ast::LocalSource with only one used variant

`LocalSource` indicated wether a let binding originated from for-loop desugaring to enable specialized error messages, but for-loop expansion has changed and this is now achieved through `MatchSource::ForLoopDesugar`.

src/libsyntax/fold.rs

@@ -515,12 +515,11 @@ pub fn noop_fold_parenthesized_parameter_data<T: Folder>(data: ParenthesizedPara
 }
 
 pub fn noop_fold_local<T: Folder>(l: P<Local>, fld: &mut T) -> P<Local> {
-    l.map(|Local {id, pat, ty, init, source, span}| Local {
+    l.map(|Local {id, pat, ty, init, span}| Local {
         id: fld.new_id(id),
         ty: ty.map(|t| fld.fold_ty(t)),
         pat: fld.fold_pat(pat),
         init: init.map(|e| fld.fold_expr(e)),
-        source: source,
         span: fld.new_span(span)
     })
 }