ChatNames::FindUserService to not take integration

What does this MR do and why?

ChatNames::FindUserService was changed in !103478 (merged) to not necessarily need an Integration to find the ChatName record.

The integration is always irrelevant (related to #357221 (closed)) and so we can refactor the class to not ever take an integration.

• #383989 (closed)
• #357221 (closed)

QA the change

This should be a pure refactor and have no functional changes.

The follow steps should work the same on master as this branch.

To QA this feature you must have access to the Ecosystem Test Slack Workspace.

1. Start a new GitPod instance and configure the GitLab for Slack (dev) app to work with it (docs)
2. Checkout this feature branch, bundle and yarn, and restart GDK (docs)
3. Choose a project, and set up the GitLab for Slack integration.
4. Once set up, these should still work:
   1. In a Slack channel of the workspace: /gitlab-dev <project path> issue show 1 should prompt you to authorize yourself.
   2. The authorization flow should work.
   3. Afterwards, /gitlab-dev <project path> issue show 1 should show an issue in the reply (or reply with a not found message if an issue with IID 1 doesn't exist).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383989 (closed)