Beyond The End - Optimization and Fixes

    Clear leftover Wither summoning blocks

    When summoning the Wither in the modpack (wither skulls on Bygone Nether's
    soul stone), some of the skulls and base blocks stayed in the world instead
    of the whole pattern being removed.

    - New WitherSummonCleanup (server, EntityJoinLevelEvent): when a freshly
      summoned Wither appears (initial invulnerability, not loaded from disk),
      remove wither skulls and #minecraft:wither_summon_base_blocks in the area
      of the T (x/z -1..+1, y 0..+2 from the Wither, which vanilla places on the
      bottom block of the T), with the break particles. Done on join and again
      one tick later.