Remove feature flag for escalating manually created incidents

What does this MR do and why?

• Related issue: #345769 (closed)
• Epic: &5716
• Blocking(ish?) MR: !82355 (merged)

This MR removes the :incident_escalations feature flag.

How to set up and validate locally

1. Create an incident in a project in GitLab Premium
2. Change the status on the incident
3. Set the policy, unset the policy (create a policy w/ maintainer+ under Monitor > Escalation Policies | docs)
4. View Monitor > Incidents to see & sort by the status column

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 #345769 (closed)