AsyncWorker: added removeFromThreadStore()

and use it instead of overwriting with null things, which still occupies memory

src/pocketmine/level/generator/GeneratorUnregisterTask.php

@@ -35,7 +35,7 @@ class GeneratorUnregisterTask extends AsyncTask{
 	}
 
 	public function onRun(){
-		$this->saveToThreadStore("generation.level{$this->levelId}.manager", null);
+		$this->removeFromThreadStore("generation.level{$this->levelId}.manager");
-		$this->saveToThreadStore("generation.level{$this->levelId}.generator", null);
+		$this->removeFromThreadStore("generation.level{$this->levelId}.generator");
 	}
 }

src/pocketmine/scheduler/AsyncTask.php

@@ -151,6 +151,18 @@ abstract class AsyncTask extends Collectable{
 		$this->worker->saveToThreadStore($identifier, $value);
 	}
 
+	/**
+	 * @see AsyncWorker::removeFromThreadStore()
+	 *
+	 * @param string $identifier
+	 */
+	public function removeFromThreadStore(string $identifier) : void{
+		if($this->worker === null or $this->isGarbage()){
+			throw new \BadMethodCallException("Objects can only be removed from AsyncWorker thread-local storage during task execution");
+		}
+		$this->worker->removeFromThreadStore($identifier);
+	}
+
 	/**
 	 * Actions to execute when run
 	 *

src/pocketmine/scheduler/AsyncWorker.php

@@ -98,4 +98,13 @@ class AsyncWorker extends Worker{
 	public function getFromThreadStore(string $identifier){
 		return self::$store[$identifier] ?? null;
 	}
+
+	/**
+	 * Removes previously-stored mixed data from the worker's thread-local object store.
+	 *
+	 * @param string $identifier
+	 */
+	public function removeFromThreadStore(string $identifier) : void{
+		unset(self::$store[$identifier]);
+	}
 }