xen: Kconfig: Drop reference to obsolete configs MCORE2 and MK8

Commit f388f60c ("x86/cpu: Drop configuration options for early 64-bit CPUs")
removes the config symbols MCORE2 and MK8.

With that, the references to those two config symbols in xen's x86 Kconfig
are obsolete. Drop them.

Fixes: f388f60c ("x86/cpu: Drop configuration options for early 64-bit CPUs")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20250303093759.371445-1-lukas.bulwahn@redhat.com