Hotfix double entity removal making entity scheduler retire call

The assumption that the setRemoved method will never be called more than once is flawed, considering even vanilla code seems to indicate it might happen. Especially with plugins, throwing an exception is not something reasonably maintainable across all the places it *could* happen.
If it is called a second time after already having been removed due to changing dimensions, that's definitely bad, so no extra check for that
Fixes #9420

patches/server/0977-Folia-scheduler-and-owned-region-API.patch

@@ -1158,7 +1158,7 @@ index 8547e7ff2f1f5b7701fb0f3c3010c14601a5f83e..fff7ad7a45f310783ac96b44575ad3db
          this.players.remove(entityplayer);
          this.playersByName.remove(entityplayer.getScoreboardName().toLowerCase(java.util.Locale.ROOT)); // Spigot
 diff --git a/src/main/java/net/minecraft/world/entity/Entity.java b/src/main/java/net/minecraft/world/entity/Entity.java
-index b38c4cbcf0405d82c7b6e018e80a3174e460c1a4..513c34aa02d63f7e3c178eade818e156af4541db 100644
+index b38c4cbcf0405d82c7b6e018e80a3174e460c1a4..6abaf7ef99800a238b29dbbb85de8c970c0806a7 100644
 --- a/src/main/java/net/minecraft/world/entity/Entity.java
 +++ b/src/main/java/net/minecraft/world/entity/Entity.java
 @@ -246,11 +246,23 @@ public abstract class Entity implements Nameable, EntityAccess, CommandSource {
@@ -1194,12 +1194,20 @@ index b38c4cbcf0405d82c7b6e018e80a3174e460c1a4..513c34aa02d63f7e3c178eade818e156
          this.remove(Entity.RemovalReason.DISCARDED);
      }
  
-@@ -4678,12 +4691,28 @@ public abstract class Entity implements Nameable, EntityAccess, CommandSource {
+@@ -4668,6 +4681,7 @@ public abstract class Entity implements Nameable, EntityAccess, CommandSource {
+             return;
+         }
+         // Paper end - rewrite chunk system
++        final boolean alreadyRemoved = this.removalReason != null;
+         if (this.removalReason == null) {
+             this.removalReason = reason;
+         }
+@@ -4678,12 +4692,28 @@ public abstract class Entity implements Nameable, EntityAccess, CommandSource {
  
          if (reason != RemovalReason.UNLOADED_TO_CHUNK) this.getPassengers().forEach(Entity::stopRiding); // Paper - chunk system - don't adjust passenger state when unloading, it's just not safe (and messes with our logic in entity chunk unload)
          this.levelCallback.onRemove(reason);
 +        // Paper start - Folia schedulers
-+        if (!(this instanceof ServerPlayer) && reason != RemovalReason.CHANGED_DIMENSION) {
++        if (!(this instanceof ServerPlayer) && reason != RemovalReason.CHANGED_DIMENSION && !alreadyRemoved) {
 +            // Players need to be special cased, because they are regularly removed from the world
 +            this.retireScheduler();
 +        }