monitor: Don't check for mon_get_cpu() failure

mon_get_cpu() can't return null pointer, because it passes its return
value to cpu_synchronize_state() first, which crashes if its argument
is null.

Remove the (pretty cheesy) handling of this non-existing error.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
