ESXi still thinks it is connected to vCenter...

and you can't remove any resource pools. I had this issue because I created an SRM lab, tried the exercises and then reverted to snapshots. While testing the lab I created resource pools which were pushed to the ESX hosts. When I reverted to the snapshots, my vCenter was resetted, but the ESX hosts still had the vcenter agent running and in effect they thought they were still connected to a vCenter. You'll also get the annoying issue that when you reconnect a host to "another" vCenter, it will complain about already being managed. When you then try to remove the resource pool, the ESXi host for some reason auto recreates the resource pool. Very annoying :) You could readadd it to a vCenter and then properly remove it. However you can also do the following

Enable the remote technical support.
Run /opt/vmware/uninstallers/VMware-vpxa-uninstall.sh

You should be able to remove the resource pool. I remembered this command from an issue I used to have when I did a migration. Here is a link to the kb article