Do not apply incident label to incidents

Summary

Once #268152 (closed) is implemented, there is no driving force for applying the incident label to newly created incidents (whether created by Alert bot or users).

Release Notes

Stop automatically applying the incident label to new incidents - this happens when incidents are created manually AND automatically by the GitLab Alert Bot.~~ We will do this after #268152 (closed) is completed, but we can do the first step now.

Implementation details

Removing Issues::BaseServce#add_incident_label & callers during creation should prevent (re-)adding the ~incident label.

• Possibly remove or relocate logic from IncidentManagement::CreateIncidentLabelService including LABEL_PROPERTIES. Usage ping should retain functionality or removal should be verified with product.
• Possibly remove usage data key counts.incident_labeled_issues.