Let the platform login control user login/register flow

[CodeWithEmad]

Forcing users to go to Authn MFE results in a bad UX. We need to let the platform decide on login and registration. Imagine this scenario: you have a TPA setup and want users to log in there directly. you can have this by adding "THIRD_PARTY_AUTH_HINT": "some-provider-slug" and if users go to /login it redirects them to the TPA login/registration page but the current logic takes users to the Authn MFE and they need to know that they have to click on another button which results in a bad UX!

https://github.com/openedx/edx-platform/blob/3c571f33c6a3654d4ea48a593aedfc69da5f9276/lms/templates/header/navbar-not-authenticated.html#L51-L69