xenpaging: remove unused spinlock in pager

xenpaging: remove unused spinlock in pager

The spinlock code in the pager is a no-op because xenpaging is a single
threaded application. There is no locking when put_response() places a
response into the ringbuffer.
The only locking is inside the hypervisor, where mem_event_put_request() and
mem_event_get_response() lock the ringbuffer to protect multiple vcpus from
each other.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Committed-by: Ian Jackson <ian.jackson.citrix.com>