[Bug] Fix bug that task_worker_pool not work (#4543)

The number of thread initialized in task worker pool is not right.
This bug is introduced from #4440

fe/fe-core/src/test/java/org/apache/doris/analysis/SelectStmtTest.java

@@ -18,6 +18,7 @@
 package org.apache.doris.analysis;
 
 import org.apache.doris.common.AnalysisException;
+import org.apache.doris.common.Config;
 import org.apache.doris.qe.ConnectContext;
 import org.apache.doris.planner.Planner;
 import org.apache.doris.qe.VariableMgr;
@@ -47,6 +48,7 @@ public class SelectStmtTest {
 
     @BeforeClass
     public static void setUp() throws Exception {
+        Config.enable_batch_delete_by_default = true;
         UtFrameUtils.createMinDorisCluster(runningDir);
         String createTblStmtStr = "create table db1.tbl1(k1 varchar(32), k2 varchar(32), k3 varchar(32), k4 int) "
                 + "AGGREGATE KEY(k1, k2,k3,k4) distributed by hash(k1) buckets 3 properties('replication_num' = '1');";