Remove AddDefaultGroupToSortOrder check from startup.

The call to AddDefaultGroupToSortOrder in NERD_tree.vim is redundant
because it's also done every time sortChildren is called. And since the
check is done only once, there's no need for a function either.
sortChildren now just contains the needed if statement.

lib/nerdtree/tree_dir_node.vim

@@ -607,7 +607,9 @@ endfunction
 " FUNCTION: TreeDirNode.sortChildren() {{{1
 " Sort "self.children" by alphabetical order and directory priority.
 function! s:TreeDirNode.sortChildren()
-    call AddDefaultGroupToSortOrder()
+    if count(g:NERDTreeSortOrder, '*') < 1
+        call add(g:NERDTreeSortOrder, '*')
+    endif
     let CompareFunc = function("nerdtree#compareNodesBySortKey")
     call sort(self.children, CompareFunc)
 endfunction