x86-64/EFI: don't call EfiResetSystem() from machine_halt()

c/s 23615:d19e77844267 was a little too eager in adding calls to EFI
runtime services: machine_halt() doesn't really want to power off the
machine, but that's what EfiResetSystem(EfiResetShutdown, ...) (called
through efi_halt_system()) does.

Signed-off-by: Jan Beulich <jbeulich@novell.com>

xen/arch/x86/shutdown.c

@@ -96,7 +96,6 @@ void machine_halt(void)
     watchdog_disable();
     console_start_sync();
     local_irq_enable();
-    efi_halt_system();
     smp_call_function(__machine_halt, NULL, 0);
     __machine_halt(NULL);
 }