Remove unused files

Remove unused (unreferenced) images. Most of these were migrated because they existed in the old repository, but wasn't checked for usage.

From report:

Unlinked file: content/handbook/support/workflows/assets/delete_1.jpg
Unlinked file: content/handbook/support/workflows/assets/delete_2.jpg
Unlinked file: content/handbook/support/workflows/assets/delete_3.jpg
Unlinked file: content/handbook/support/workflows/assets/delete_4.jpg
Unlinked file: static/images/support/cmoc_incident-slack-thread.jpg
Unlinked file: static/images/support/cmoc_maintenance_details_automation.jpg
Unlinked file: static/images/support/organization_page_details.jpg
Unlinked file: static/images/support/organization_page_fields.jpg
Unlinked file: static/images/support/organization_page_name.jpg
Unlinked file: static/images/support/organization_page_tickets.jpg
Unlinked file: static/images/support/organization_page_users.jpg

Part of #274 (moved)