Drop the HipChat plugin

https://en.wikipedia.org/wiki/HipChat says:
> Following this, HipChat and Stride customers were migrated to the
> Slack group collaboration platform in a transition that was completed by
> February 2019.

doc/manual/src/plugins/README.md

@@ -172,17 +172,6 @@ Sets Gitlab CI status.
 
 - `gitlab_authorization.<projectId>`
 
-## HipChat notification
-
-Sends hipchat chat notifications when a build finish.
-
-### Configuration options
-
-- `hipchat.[].jobs`
-- `hipchat.[].builds`
-- `hipchat.[].token`
-- `hipchat.[].notify`
-
 ## InfluxDB notification
 
 Writes InfluxDB events when a builds finished.