Remove unused images

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/customer-success/csm/segment/cse/cse-help.gif
Unlinked file: content/handbook/customer-success/professional-services-engineering/operational_services1.jpg
Unlinked file: content/handbook/customer-success/professional-services-engineering/operational_services2.jpg
Unlinked file: content/handbook/customer-success/professional-services-engineering/product_services1.jpg
Unlinked file: content/handbook/customer-success/professional-services-engineering/product_services2.jpg
Unlinked file: content/handbook/customer-success/professional-services-engineering/sow-approvals-board.jpg
Unlinked file: content/handbook/customer-success/professional-services-engineering/strategic_services.jpg

Part of #274 (moved)