Remove postgresql-xl (unmaintained)

- no commits since 2018 https://git.postgresql.org/gitweb/?p=postgres-xl.git;a=summary
- only compatible with postgres 9/10

README.md

@@ -578,7 +578,6 @@ Comparison of NoSQL servers: http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-v
   * [adminer](https://www.adminer.org/) -  Database management in a single PHP file ([Source code](https://github.com/vrana/adminer))
 * [Percona Server](https://www.percona.com/software) - Enhanced, drop-in MySQL replacement.
 * [PostgreSQL](http://www.postgresql.org/) - Object-relational database management system (ORDBMS).
-* [PostgreSQL-XL](http://www.postgres-xl.org/) - Scalable PostgreSQL-based database cluster.
 * [SQLite](http://sqlite.org/) - Library that implements a self-contained, serverless, zero-configuration, transactional SQL DBS.
 
 ### Remote Management