## Problem
Test `test_metric_collection` become flaky:
```
AssertionError: assert not ['2023-05-25T14:03:41.644042Z ERROR metrics_collection: failed to send metrics: reqwest::Error { kind: Request, url: Url { scheme: "http", cannot_be_a_base: false, username: "", password: None, host: Some(Domain("localhost")), port: Some(18022), path: "/billing/api/v1/usage_events", query: None, fragment: None }, source: hyper::Error(Connect, ConnectError("tcp connect error", Os { code: 99, kind: AddrNotAvailable, message: "Cannot assign requested address" })) }',
...]
```
I suspect it is caused by having 2 places when we define
`httpserver_listen_address` fixture (which is internally used by
`pytest-httpserver` plugin)
## Summary of changes
- Remove the definition of `httpserver_listen_address` from
`test_runner/regress/test_ddl_forwarding.py` and keep one in
`test_runner/fixtures/neon_fixtures.py`
- Also remote unused `httpserver_listen_address` parameter from
`test_proxy_metric_collection`
## Describe your changes
## Issue ticket number and link
## Checklist before requesting a review
- [x] I have performed a self-review of my code.
- [x] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.
