Evaluate restoring using a form alter instead of extending UserLoginForm

Problem/Motivation

In 34a6ca4dd5cce9e375916e50cd5b6956007918ff the code was re-factored from using a form alter to extending the UserLoginForm.

Around 5 years ago the class was marked @internal in ee0f806896ee167f382935bfb40cefec9d4ad1cb

If reverting to a form alter would allow us not to extend core classes it could make the code much more managble and allow us to reduce the maintenance burden.

Steps to reproduce

Review Code

Proposed resolution

Revert to a form alter, or form alter+service configuration.

Remaining tasks

Patch

User interface changes

None

API changes

Impacted classes are @internal.
Remove TfaUserLogin and and only decorate with form alter and validation calls

Data model changes

None