xen/x86: Drop sync_core()

xen/x86: Drop sync_core()

As identified in Linux c/s c198b121b1a1d "x86/asm: Rewrite sync_core() to use
IRET-to-self", sync_core() is only appropriate for two very specific usecases.

Xen doesn't have need of either of these usecases, so drop sync_core() to
avoid any misuse.

In the unlikely event that we do gain a legitimate use for sync_core(), it
should be reintroduced as a mov to %cr2 rather than cpuid, which has a lower
overhead.

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