Released to GA the theme parameter in the /authentication/setup route

refs daeb06e835

- This is an additive change, and the parameter is optional so it's better to ship it right away (testing was hard as it's a chicken and an egg problem to have a feature flag in the setup route).

core/server/api/canary/authentication.js

@@ -56,10 +56,7 @@ module.exports = {
                     }
                 })
                 .then((data) => {
-                    if (labs.isSet('improvedOnboarding')) {
-                        return auth.setup.installTheme(data, api);
-                    }
-                    return data;
+                    return auth.setup.installTheme(data, api);
                 })
                 .then((data) => {
                     return auth.setup.doSettings(data, api.settings);

test/regression/api/admin/authentication.test.js

@@ -33,9 +33,6 @@ describe('Authentication API', function () {
         });
 
         it('complete setup', async function () {
-            // Enable the improvedOnboarding flag
-            mockManager.mockLabsEnabled('improvedOnboarding');
-
             await agent
                 .post('authentication/setup')
                 .body({