[chores] Wire openwisp-users password reset serializer into REST_AUTH when openwisp-radius is absent

[pandafy]

Describe the bug
After the password reset serializer lands in openwisp-users, docker-openwisp still needs a follow-up change in settings.py to use openwisp-users' PASSWORD_RESET_SERIALIZER in REST_AUTH settings when openwisp-radius is not installed.

Steps To Reproduce

1. Run a docker-openwisp deployment without openwisp-radius installed.
2. Inspect the REST authentication settings used by settings.py.
3. Notice that the password reset serializer is not yet wired from openwisp-users.

Expected behavior
settings.py should configure REST_AUTH to use PASSWORD_RESET_SERIALIZER from openwisp-users whenever openwisp-radius is not installed, so the new password reset flow works consistently.

Screenshots
N/A

System Informatioon:

• OS: N/A
• Docker version: N/A
• Browser and Browser Version (if applicable): N/A

[nemesifier]

This is not needed anymore because OpenWISP Users defines the password reset serializer directly, without relying on REST_AUTH.