Invitation links show registration step when registration link is disabled

Description:

Currently, the registration link is disabled and it's available only via OAuth provider.

Via the normal login page https://chat.example.com it's not showing as expected, but when I send an invitation link to someone it appears.

Steps to reproduce:

1. Add an OAuth provider
2. Disable all the other authentication/registration methods
3. Send an invitation link to someone

Expected behavior:

To see the login via oauth provider only, like the main login page:

Actual behavior:

I'm seeing the login and a registration link right below:

Server Setup Information:

• Version of Rocket.Chat Server: 3.5.4
• Operating System: Ubuntu 20.04
• Deployment Method: docker-compose
• Number of Running Instances: 1
• DB Replicaset Oplog: Enabled
• NodeJS Version: v12.16.1
• MongoDB Version: 4.0.20

Client Setup Information

• Desktop App or Browser Version: Google Chrome | 84.0.4147.135 (Official Build) (64-bit)
• Operating System: Mac OS 10.15.6

Reproduce the invitation-link flow with an OAuth provider enabled and all other authentication or registration methods disabled, then compare it with the normal login page. Done means the invitation link offers only the OAuth login, without the registration link shown in the reported screenshots.