Remove manual instruction encodings for VMLOAD, VMRUN, and VMSAVE.

This is a relic from when these instructions weren't supported by the toolchain.
No functional change.

Submitted by: adam_fenn.io
Reviewed by: grehan
Approved by: grehan (bhyve)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D27130