x86/spec-ctrl: Drop use_spec_ctrl boolean

x86/spec-ctrl: Drop use_spec_ctrl boolean

Several bugfixes have reduced the utility of this variable from it's original
purpose, and now all it does is aid in the setup of SCF_ist_wrmsr.

Simplify the logic by drop the variable, and doubling up the setting of
SCF_ist_wrmsr for the PV and HVM blocks, which will make the AMD SPEC_CTRL
support easier to follow.  Leave a comment explaining why SCF_ist_wrmsr is
still necessary for the VMExit case.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit ec083bf552c35e10347449e21809f4780f8155d2)