Mercedes: Fix Refresh token handling

[ReneNulschDE]

With the latest API changes made by Mercedes, the refresh token is stable and is not changed anymore with every token refresh. (Not sure if this is a config bug on MB side as this will reduce the security level) - this PR will remove all the code that handled the rolling refresh token logic.

Now, the refresh token out of the config is the only source of true.

Setting store usage is removed. No need to delete of tokens in the db anymore...

User account is not needed anymore, for each car a separate refresh token is needed now. --> breaking change...

The new code was running in my environment for multiple hours and multiple restarts...

@andig: May I ask you to check it with your knowledge of evcc. Thx. I planned to remove the not needed access token in the config, but not sure why it is still visible in the frontend. (We need the refresh token only, as we request a new access token on restart.)

fixes: #18863

[ReneNulschDE]

I keep it as draft, as first would like the feedback from andig...

[andig]

I don‘t understand why the code wouldn‘t work without this change.

[ReneNulschDE]

the old code expected that a new refresh token is delivered by the api on token expiry. this is not the case anymore. The new answer is just an access token...

[ReneNulschDE]

I could simplify the PR by just adding the old RT to the result set, but this would lead to 60 lines unused code and the "unneeded" usage of the settings store. (with all the "token deletion" questions)

[andig]

In den anderen Integrationen nehmen wir den neuen Refreshtoken einfach für den Fall, dass es einen gibt. Sonst bleibt der alte erhalten. Beide müssten gespeichert werden.