menubar.show(): Remove duplicate call to menubar.refresh()

This changes a line of code that was added in the previous commit.
Previously, when menubar.cache_entries was set to false,
menubar.refresh() was called twice.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/menubar/init.lua

@@ -388,12 +388,15 @@ end
 -- @param scr Screen.
 function menubar.show(scr)
     if not instance then
-        menubar.refresh(scr)
         -- Add to each category the name of its key in all_categories
         for k, v in pairs(menubar.menu_gen.all_categories) do
             v.key = k
         end
 
+        if menubar.cache_entries then
+            menubar.refresh(scr)
+        end
+
         instance = {
             wibox = wibox({ ontop = true }),
             widget = common_args.w,