Actually, you need to do a 'conversion' which is simple. Just install vCenter Converter on your workstation then convert the Server VM to ESXi.
Converter Tool:
https://www.vmware.com/tryvmware/?p=converterConverter User's Guide (pg. 37):
http://www.vmware.com/pdf/converter_standalone_guide401.pdfRegards,
~coolsport00