Remove dead code from directory_tree.js

This function isn't being called by any other code, its user was removed
in crrev.com/c/1203596

Change-Id: I78e75f7720bfcabea3cf4047c4fb5fab612ccaaa
Reviewed-on: https://chromium-review.googlesource.com/c/1301033Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602994}

ui/file_manager/file_manager/foreground/js/ui/directory_tree.js

@@ -428,23 +428,6 @@ DirectoryItem.prototype.onExpand_ = function(e) {
   e.stopPropagation();
 };
 
-/**
- * Returns all (recursive) visible children of parentItem.
- * @param {DirectoryItem} parentItem The parent item to start searching from.
- * @return {!Array<Entry>}
- */
-let getVisibleChildEntries = (parentItem) => {
-  if (parentItem.expanded == false) {
-    return [parentItem.entry];
-  }
-  let visibleChildEntries = [];
-  for (let childItem of parentItem.items) {
-    visibleChildEntries.concat(
-        visibleChildEntries, getVisibleChildEntries(childItem));
-  }
-  return visibleChildEntries;
-};
-
 /**
  * Invoked when the item is being collapsed.
  * @param {!Event} e Event.