Discontinue support for ReviewDb monitoring

Upcoming gerrit release will remove ReviewDb binding code. Do the same
in this plugin. This will allow to simplify plugin build and deployment.
Because we don't need any more the split of plugin in two artifacts, one
to deploy in `gerrit_site`/lib and one to `gerrit_site`/plugins.

Also update bazlets to use 3.0-SNAPSHOT plugin API.

Change-Id: Ie542d297093ee79f57dbffb5d3bedeb06b8dd5fd
6 files changed