/usr/lib/python2.7/dist-packages/charmtools/templates/bash/files/hooks/relation-name-relation-broken is in charm-tools 2.1.2-0ubuntu4.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 | #!/bin/sh
# This hook runs when the full relation is removed (not just a single member)
|