In order to migrate to the latest release of rabbitmq (3.7), we need to
first remove this deprecated plugin which is no longer supported (the
problems it solved are now addressed in rabbitmq itself).

This avoids a circular dependency in CI where the new images depend on
the new clustering and the new clustering depends on the new images.

Change-Id: I921459f3e40b9e0d4af9497384e49aabf0abe79b