openvswitch-switch.postinst: Remove upstart files.

openvswitch-switch.postinst: Remove upstart files.

For a while, ubuntu used upstart scripts as an alternative
for sysv scripts. For e.g popular releases like ubuntu12.04
and ubuntu14.04 have upstart infrastructure. From Ubuntu16.04,
they have moved to systemd.

Ubuntu maintainers have a fork of openvswitch packages with
upstart scripts. When a user installs openvswitch-switch
package on a ubuntu 14.04 with 'apt-get install openvswitch-switch',
two upstart files are added to /etc/init directory.

Now, when the same user builds debian packages from this repo
and then installs them, the older upstart files don't get
deleted. What this means is that a command like
'invoke-rc.d openvswitch-switch restart' calls initctl scripts
instead of sysv script that this repo provides. In our sysv
scripts we have logic to carefully upgrade kernel module and
save openflow flows. This breaks the intent of the package from
this repo.

This commit deletes any stale upstart files that have remained
in the system.

Signed-off-by: Gurucharan Shetty <guru@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>