Add delete button in edit more for incident timeline events

An additional item was flagged as missing from !95061 (merged):

There are a couple things missing that I see in the design:

• Ability to Delete from edit mode

We should add this in to ensure the implementation matches the design.

Implementation guide

Files to look at:

• app/assets/javascripts/issues/show/components/incidents/edit_timeline_event.vue
• app/assets/javascripts/issues/show/components/incidents/timeline_events_form.vue
• spec/frontend/issues/show/components/incidents/edit_timeline_event_spec.js
• spec/frontend/issues/show/components/incidents/timeline_events_form_spec.js

Additional guidance:

1. Ensure you are running enterprise edition
2. Go to a project's settings, then Monitor
3. Create an incident or open an existing one
4. Navigate to Timeline tab
5. Toggle a dropdown button on a timeline item and select Edit