xenserver: Delete network.conf on package removal on XenServer 5.5.0.

xenserver: Delete network.conf on package removal on XenServer 5.5.0.

The previous commit "xenserver: Create network.conf before running
interface-reconfigure" fixed a problem that was not noticed earlier only
because we had not tested very often on pristine XenServer 5.5.0 systems,
instead mostly on systems that had had Open vSwitch installed before and
thus already had a network.conf file.  This commit should help to avoiding
future regressions in this area, by removing network.conf when the
openvswitch package is removed on XenServer 5.5.0.