Add audit event on webhook deletion

Problem

Audit Events are a tool for GitLab owners and administrators to view records of important actions performed across the application.

At the moment GitLab owners and administrators don't have insights into webhook deletions.

Proposed solution

Add audit event for webhook deletion, so that GitLab owners and administrators can have insight into who and when deleted webhooks.

Follow Audit event development guide: https://docs.gitlab.com/ee/development/audit_event_guide/