nit: Remove unused stub function

[syou6162]

resolves N/A

Problem

The _stub_relation function is not used anywhere. The increase in unused code increases the cognitive load on the developer.

Solution

Delete the _stub_relation function, which is not used anywhere.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[syou6162]

The CI for Check Changelog Entry has failed, but since it only deletes unused code, I have decided that the ChangeLog can be skipped.

[mikealfare]

@syou6162 Thanks for the "maintenance" PR. Not everyone is keen to submit those, but they're just as valuable to us.

[mikealfare]

Integration tests didn't trigger. Re-opening to re-trigger CI.