Remove redundant call to worldMaps.a(). Addresses BUKKIT-4828

The WorldMapCollection object for SecondaryWorldServers(Nether, End) is
shared with the main world, so saving it again for each SecondaryWorldServer
is redundant.

This commit removes the redundant call by checking if the WorldServer is an
instanceof SecondaryWorldServer before invoking worldMaps.a().
Merge remote-tracking branch 'upstream/master' into slow-hopper-world