Remove "Auto fix bad Y levels on player login" patch

Fixes #6357
Closes #6508
Closes #6358

patches/server/0412-Fix-Chunk-Post-Processing-deadlock-risk.patch

@@ -0,0 +1,59 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Sat, 18 Apr 2020 04:36:11 -0400
+Subject: [PATCH] Fix Chunk Post Processing deadlock risk
+
+See: https://gist.github.com/aikar/dd22bbd2a3d78a2fd3d92e95e9f28dc6
+
+as part of post processing a chunk, we can call ChunkConverter.
+
+ChunkConverter then kicks off major physics updates, and when blocks
+that have connections across chunk boundries occur, a recursive risk
+can occur where A updates a block that triggers a physics request.
+
+That physics request may trigger a chunk request, that then enqueues
+a task into the Mailbox ChunkTaskQueueSorter.
+
+If anything requests that same chunk that is in the middle of conversion,
+it's mailbox queue is going to be held up, so the subsequent chunk request
+will be unable to proceed.
+
+We delay post processing of Chunk.A() 1 "pass" by re stuffing it back into
+the executor so that the mailbox ChunkQueue is now considered empty.
+
+This successfully fixed a reoccurring and highly reproduceable crash
+for heightmaps.
+
+diff --git a/src/main/java/net/minecraft/server/level/ChunkMap.java b/src/main/java/net/minecraft/server/level/ChunkMap.java
+index a586623ed0f7003905e053c238d9b62632b87bc6..205d6b9d822d3d28c9e8c3f5a511384ce82adc99 100644
+--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
++++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
+@@ -168,6 +168,7 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
+     };
+     // CraftBukkit end
+ 
++    final CallbackExecutor chunkLoadConversionCallbackExecutor = new CallbackExecutor(); // Paper
+     // Paper start - distance maps
+     private final com.destroystokyo.paper.util.misc.PooledLinkedHashSets<ServerPlayer> pooledLinkedPlayerHashSets = new com.destroystokyo.paper.util.misc.PooledLinkedHashSets<>();
+     // Paper start - no-tick view distance
+@@ -1056,7 +1057,7 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
+                 return Either.left(chunk);
+             });
+         }, (runnable) -> {
+-            this.mainThreadMailbox.tell(ChunkTaskPriorityQueueSorter.message(holder, runnable));
++            this.mainThreadMailbox.tell(ChunkTaskPriorityQueueSorter.message(holder, () -> ChunkMap.this.chunkLoadConversionCallbackExecutor.execute(runnable))); // Paper - delay running Chunk post processing until outside of the sorter to prevent a deadlock scenario when post processing causes another chunk request.
+         });
+ 
+         completablefuture1.thenAcceptAsync((either) -> {
+diff --git a/src/main/java/net/minecraft/server/level/ServerChunkCache.java b/src/main/java/net/minecraft/server/level/ServerChunkCache.java
+index c7ecb8fb1cbc1433986fbd03c68b4dbe5a2835c7..abd814f397a90fcaaf98a76ed83b5f9f76f2948e 100644
+--- a/src/main/java/net/minecraft/server/level/ServerChunkCache.java
++++ b/src/main/java/net/minecraft/server/level/ServerChunkCache.java
+@@ -1136,6 +1136,7 @@ public class ServerChunkCache extends ChunkSource {
+                 return super.pollTask() || execChunkTask; // Paper
+             }
+         } finally {
++            chunkMap.chunkLoadConversionCallbackExecutor.run(); // Paper - Add chunk load conversion callback executor to prevent deadlock due to recursion in the chunk task queue sorter
+             chunkMap.callbackExecutor.run();
+         }
+         // CraftBukkit end