Remove translation editor from ACP

I think it's just the same as with styles and template files. If people want to change them, they should just download them. The editor also changes the number of tabs in the file, which results in a nice conflict on the next phpBB update. Also adding support to add/remove plural rules will result in way more effort then we should invest.

Also see https://github.com/phpbb/phpbb/pull/2289 for more information