Remove tasks from lookup map upon completion

proxy/src/main/java/com/velocitypowered/proxy/scheduler/VelocityScheduler.java

@@ -187,6 +187,7 @@ public class VelocityScheduler implements Scheduler {
         } catch (Exception e) {
           Log.logger.error("Exception in task {} by plugin {}", runnable, plugin, e);
         } finally {
+          onFinish();
           currentTaskThread = null;
         }
       });