Revert "Require client_id in /user.json endpoint"

Licensify hasn't been updated to support this new behaviour yet.

This reverts commit 7a313548c47e5d8b0ea8c3b648bb6901e962e4e7.
Merge pull request #286 from alphagov/restore_deprecated_authentication_behaviour

Revert "Require client_id in /user.json endpoint"