remove server listener timeout

should be needed, we're setting the timeout on the ES proxy route, and this mixes concerns

Fixes #6377

src/server/KbnServer.js

@@ -73,7 +73,6 @@ module.exports = class KbnServer {
     let { server, config } = this;
 
     await this.ready();
-    server.listener.timeout = config.get('elasticsearch.requestTimeout');
     await fromNode(cb => server.start(cb));
     await require('./pid')(this, server, config);