tools/hotplug: remove EnvironmentFile from xen-qemu-dom0-disk-backend.service

tools/hotplug: remove EnvironmentFile from xen-qemu-dom0-disk-backend.service

The referenced Environment file does not exist, and the service file
does not make use of variables anyway.

N.B. If we start honouring env settings for any reason this will
have to be changed.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>