x86/amd: revert commit 944e0fc5

Revert commit 944e0fc5 ("x86/amd: don't
set X86_BUG_SYSRET_SS_ATTRS when running under Xen") as it is lacking
a prerequisite patch and is making things worse.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>