remove pgobserver (unmaintained since 2020) (#446)

- the project is archived

README.md

@@ -567,7 +567,6 @@ _See also: [IT Asset Management]([Ralph](#it-asset-management))_
   - [Collectd](http://collectd.org/) - System statistic collection daemon.
   - [Collectl](http://collectl.sourceforge.net/) - High precision system performance metrics collecting tool.
   - [Diamond](https://github.com/python-diamond/Diamond) - Python based statistic collection daemon.
-  - [PGObserver](https://github.com/zalando/PGObserver) - Monitoring solution for PostgreSQL databases that also works with AWS RDS.
   - [Statsd](https://github.com/etsy/statsd/) - Application statistic listener.
   - [tcollector](http://opentsdb.net/docs/build/html/user_guide/utilities/tcollector.html) - System statistic collection daemon written in Python for OpenTSDB
   - [Telegraf](https://github.com/influxdata/telegraf) - The plugin-driven server agent for collecting & reporting metrics.