amd64/vmm: Remove useless global variables

No functional change intended.

Reviewed by: corvink, jhb, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D53420

(cherry picked from commit 3faad9e26018dfc0ab6da21a53c0310656187d7b)