This commit fixes the following issue and also improves the way the code
is handling write/clean hrmc tokens on user.

In the write/clear functions, `user` argument is now mandatory so that
we can do the operation on the selected user rather than the environment
user.

Issue:

Step to reproduce:

- install`l10n_uk_reports`
- create 2 user or more
- for each user, fill their hmrc token values
- select a user different from the current environment user
- remove their credential with the button "Remove Authentication Credentials"

Current behavior:

- The selected user's credential is not removed, but the environment
  user one got removed

Expected behavior:

- the selected user's credential should be the one that is removed, and
  the environment user should not be removed.

Why it happens:

- In `_clean_tokens` function, it clears the credential of the
  environment user by default.

Solution:

- change the signature of _clean_tokens function to have `user` required
  argument and clean that user's token instead of `env.user`

opw-4041604