Simplify translation template

`this.po` is not being used anywhere, so drop the whole `!loaded` code
path. There is no defer mechanism, so `po.js` *must* be loaded after
cockpit.js. Let's trigger a crash instead of silently dropping
translations, to make such a bug obvious in manual and automatic tests.

po/po.empty.js

@@ -1,14 +1,2 @@
-(function (root, data) {
+/* The syntax of this line is important for po2json */
-    var loaded, module;
+cockpit.locale({"":{"language":"en"}});
-
-    /* Load into Cockpit locale */
-    if (typeof cockpit === 'object') {
-        cockpit.locale(data)
-        loaded = true;
-    }
-
-    if (!loaded)
-        root.po = data;
-
-/* The syntax of this line is important  by po2json */
-}(this, {"":{"language":"en"}}));