vmm: remove unneccessary rendezvous assertion

When a vcpu sees that a rendezvous is in progress, it exits and tries to
handle the rendezvous. The vcpu doesn't check if it's part of the
rendezvous or not. If the vcpu isn't part of the rendezvous, the
rendezvous could be done before it reaches the assertion. This will
cause a panic.

The assertion isn't needed at all because vm_handle_rendezvous properly
handles a spurious rendezvous. So, we can just remove it.

PR: 267779
Reviewed by: jhb, markj
Tested by: bz
Approved by: manu (mentor)
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D37417

(cherry picked from commit fde8ce889201bf7fe86d7a3b3dfe2abf27cd2d73)