refactor: Remove deprecated call to OC_User::useBackend

[CarlSchwan]

• Resolves: #
• Target version: main

Summary

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required

[github-actions]

🐢 Performance warning.
It looks like the query count of the integration tests increased with this PR.
Database query count is now 96014 was 93102 (+3.12%)
Please check your code again. If you added a new test this can be expected and the base value in tests/integration/base-query-count.txt can be increased.

[github-actions]

🐢 Performance warning.
It looks like the query count of the integration tests increased with this PR.
Database query count is now 95998 was 93102 (+3.11%)
Please check your code again. If you added a new test this can be expected and the base value in tests/integration/base-query-count.txt can be increased.