docs: remove docs for 3rd-party GQL plugin (#9181)

The GQL plugin looks very old and the docs have syntax issues.

The docs currently at https://pouchdb.com/gql.html match those at https://github.com/pouchdb/GQL, which is also linked.

This is currently the only 3rd-party plugin with docs hosted on pouchdb.com.

Author: alxndrsn

docs/external.md

@@ -58,7 +58,7 @@ Fully replicate two PouchDB/CouchDB databases, preserving absolutely all revisio
 
 #### [PouchDB GQL](https://github.com/pouchdb/GQL)
 
-Google Query Language (GQL) queries with PouchDB. ([Documentation]({{ site.baseurl }}/gql.html))
+Google Query Language (GQL) queries with PouchDB.
 
 #### [PouchDB Hoodie API](https://github.com/hoodiehq/pouchdb-hoodie-api)