(core) Remove a defunct URL constant and a product flavor.

Summary:
The URL /create-team-site on marketing site is unused, and no longer pointing to
anything functional.

The "efcr" product flavor has been defunct for a long time. Remove
references to it.

Test Plan: No tests should be affected

Reviewers: JakubSerafin

Reviewed By: JakubSerafin

Subscribers: JakubSerafin

Differential Revision: https://phab.getgrist.com/D3890

app/server/lib/FlexServer.ts

@@ -1338,7 +1338,6 @@ export class FlexServer implements GristServer {
 
   public addNotifier() {
     if (this._check('notifier', 'start', 'homedb')) { return; }
-    // TODO: Disable notifications for Nioxus orgs, until they are ready to deal with them.
     // TODO: make Notifier aware of base domains, rather than sending emails with default
     // base domain.
     // Most notifications are ultimately triggered by requests with a base domain in them,