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

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

Since 68e1183411be ("libxc: introduce a xc_dom_arch for hvm-3.0-x86_32
guests"), HVM guests are built using XEN_DOMCTL_sethvmcontext, which
ends up disabling stdvga caching because of arch_hvm_load() being
involved in the processing of the request. With that the field is
useless, and can be dropped. Drop the helper functions manipulating /
checking as well right away, but leave the use sites of
stdvga_cache_is_enabled() with the hard-coded result the function would
have produced, to aid validation of subsequent dropping of further code.

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