Update core plugin revisions to remove Maven support

Gerrit and core plugins are now built with buck.  The Gerrit
build no longer supports Maven; all the pom.xml files have
already been removed.

Remove Maven build support, i.e. the pom.xml files, from all
core plugins except the cookbook plugin which should keep it as
long as we still actually support Maven-based plugins.

Change-Id: Id299ade79ae1f94976c9b18634aa4b458bf1eeff
3 files changed