xen: remove PV suspend/resume support copyright

Thew code for PV suspend/resume support has long been removed, also remove the
copyright notice associated with it.

There are still two copyright blocks with (to my understanding) slightly
different wordings of the BSD 2 clause license. I however don't feel like
merging them due to those wording differences.

The removal of the PV suspend/resume code was done in
ed95805e90ec0f61683cd402a42e6f915339de7d.

Sponsored by: Cloud Software Group
Reviewed by: imp
Differential revision: https://reviews.freebsd.org/D46860