Remove threat monitoring sidebar option and routes

What does this MR do and why?

Remove threat monitoring sidebar option and routes

• remove threat monitoring sidebar option
• remove threat monitoring routes
• update tests for removal

Changelog: removed

EE: true

What does this MR NOT do and why?

It does NOT delete all the underlying components (e.g. any additional controllers, views, UI code) because doing that in this MR would make this MR >2000lns long. This MR only ensures that a user can not get to any of the threat monitoring paths. All the underlying components will be deleted iteratively in follow-up MRs (coming immediately after this gets merged), but there are just so many that it will be easier to delete this first. This is all documented in #352287 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Upload a GitLab Ultimate license
2. Navigate to a project => Security & Compliance
3. Verify Threat Monitoring no longer exists
4. Verify the url /threat_monitoring is inaccessible
5. Verify the url /threat_monitoring/alerts/1#/overview is inaccessible (relies on having alerts set up)

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 #352287 (closed)