Brought back TAB to switch team/all chat when nothing has been typed yet

The new ALT hotkey from https://github.com/OpenRA/OpenRA/pull/6281 is driving me nuts. When you have not typed anything and therefore nothing can be auto-completed, this will allow you to still use it to switch team and public chat.
IMO we should remove this completely.  The main complain about having alt for toggle is that it would be triggered when a windows player alt-tabs out of the game.