x86/msr: Drop stale comment for vcpu_msrs.spec_ctrl

x86/msr: Drop stale comment for vcpu_msrs.spec_ctrl

More than the bottom two bits are now defined, and the MSR policy work has
shown that using non-architectural representations turns out to be problematic
for more than just asm code.  As the architectural representation is the
expected default, we don't need to justify why we are using it.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>