Bug 1468379 - Remove unneeded code added for Thunderbird in bug 1458682 in test_webextension_theme.js. r=aswan

--HG--
extra : rebase_source : 5a1ca98e123034845d3ecd6c92e169b01739fb0f

toolkit/mozapps/extensions/test/xpcshell/test_webextension_theme.js

@@ -234,11 +234,6 @@ add_task(async function uninstall_offers_undo() {
 add_task(async function default_locale_themes() {
   let addon = await promiseInstallWebExtension({
     manifest: {
-      applications: {
-        gecko: {
-          id: "locale-theme@tests.mozilla.org",
-        }
-      },
       default_locale: "en",
       name: "__MSG_name__",
       description: "__MSG_description__",