REST API: Remove ip_address field from RunnerDetails
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=415159)
</details>
<!--IssueSummary end-->
#### Description
`ip_address` for an individual GitLab runner doesn't make sense now when we can have multiple runner managers assigned to the same runner with different IP's.
issue