Instead of clearing the inventory of players and storing it's contents in a file, we now prevent
the server from sending the inventory packet if the player is not logged in. The player will
see a empty inventory, but has still his items stored on the server. Therefore we don't
need to modify the player's inventory and we won't make any inventory corrupted.
FixesXephi/AuthMeReloaded#203,
FixesXephi/AuthMeReloaded#193,
FixesXephi/AuthMeReloaded#191,
FixesXephi/AuthMeReloaded#148
Remove dead code + Fix empty inventory on the unregister command
Fix NPE if ProtocolLib isn't enabled or installed
