x86/HVM: drop stdvga's "stdvga" struct member

x86/HVM: drop stdvga's "stdvga" struct member

Two of its consumers are dead (in compile-time constant conditionals)
and the only remaining ones are merely controlling debug logging. Hence
the field is now pointless to set, which in particular allows to get rid
of the questionable conditional from which the field's value was
established (afaict 551ceee97513 ["x86, hvm: stdvga cache always on"]
had dropped too much of the earlier extra check that was there, and
quite likely further checks were missing).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>