prerm is in openvswitch-switch-dpdk 2.5.5-0ubuntu0.16.04.2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | #!/bin/sh
set -e
update-alternatives --remove ovs-vswitchd /usr/lib/openvswitch-switch-dpdk/ovs-vswitchd-dpdk
exit 0
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