docs: remove qemu-upstream_howto_use_it.markdown

docs: remove qemu-upstream_howto_use_it.markdown

QEMU upstream has been the default for several releases. The only
setting described in that document is no longer required.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Tim Deegan <tim@xen.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>