Refactor `User`: `authenticationProviders`, `secondFactorProviders` and `validAuthenticationMethods`

[phorward]

During development of #1125 I found that the definition of the authentication methods in user.py is split into the attributes

• authenticationProviders
• secondFactorProviders
• validAuthenticationMethods

but at least, the only true definition should be just in validAuthenticationMethods; Why should I register an auth_userpassword-login, when validAuthenticationMethods does only define GoogleLogin?

This is a relict and bad extension of old code base, and must be refactored.