timer: last, remove last bits of last

The reset notifiers kept a 'last' counter to notice jumps;
now that we've remove the notifier we don't need to keep 'last'.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190724115823.4199-5-dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>