docs: Remove xen-api docs

docs: Remove xen-api docs

This document is about an old unmaintained version of the XenAPI,
which bears little to no relation to what is implemented in xapi and
which is only partially implemented in xend (which is deprecated). The
doc hasn't seen much in the way of updates since 2009.

Anyone who is actually interested can continue to use the version
which was in 4.2.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>