[Bungee] Change PlayerDisconnectEvent priority to LOWEST

[karer]

Setting server to null could cause problems with other plugins. E.g. with auth, which stores player's latest server in database. It has normal priority and gets current server as 'null', because of BungeeTabListPlus.

This PR:

1. Changes event priority to lowest,
2. Removes manual disconnection

... so other plugins get valid info about current server.

[CodeCrafter47]

The code you remove is there as a workaround for the following BungeeCord bug: SpigotMC/BungeeCord#1601.

[CodeCrafter47]

I'd be okay with the priority change if that helps.

[Phoenix616]

Just so that nobody pulls this without considering this: Changing the priority from HIGHEST (which means run last) to LOWEST (runs first) would actually cause the issues described as now plugins listening on any higher level (LOW, NORMAL/none specified, HIGH, HIGHEST) would all listen after BTLP disconnects the player. So changing the priority would not solve anything and as it stands right now any plugin listening on anything below HIGHEST should not run into any issues? If they are occurring then someone should provide some example code that actually runs into those issues.