Temporarily revert "amd/msr: implement VIRT_SPEC_CTRL for HVM guests using legacy SSBD"

A person tagged in commit 646589ac14
claims the tag is in accurate; revert this commit so that we can
re-commit it again with the tag corrected.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>

CHANGELOG.md

@@ -13,7 +13,6 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ### Added / support upgraded
  - IOMMU superpage support on x86, affecting PV guests as well as HVM/PVH ones
    when they don't share page tables with the CPU (HAP / EPT / NPT).
- - Support VIRT_SSBD feature for HVM guests on AMD.
 
 ### Removed / support downgraded
  - dropped support for the (x86-only) "vesa-mtrr" and "vesa-remap" command line options