[fix](group commit) Fix group commit error log when decommission (#39899)

Decommission backend will check the group commit wal num is 0.
But there is a small bug of log:
```
boolean hasWal = checkWal(backend);  // the hasWal should be empty wal

hasWal ? "; and has unfinished WALs" : "" // so if log print 'and has unfinished WALs', the wal is 0 actually
```

Author: mymeiyi

fe/fe-core/src/main/java/org/apache/doris/alter/SystemHandler.java

@@ -95,8 +95,8 @@ private void runAlterJobV2() {
             }
 
             List<Long> backendTabletIds = invertedIndex.getTabletIdsByBackendId(beId);
-            boolean hasWal = checkWal(backend);
-            if (Config.drop_backend_after_decommission && checkTablets(beId, backendTabletIds) && hasWal) {
+            long walNum = Env.getCurrentEnv().getGroupCommitManager().getAllWalQueueSize(backend);
+            if (Config.drop_backend_after_decommission && checkTablets(beId, backendTabletIds) && walNum == 0) {
                 try {
                     systemInfoService.dropBackend(beId);
                     LOG.info("no available tablet on decommission backend {}, drop it", beId);
@@ -109,7 +109,7 @@ private void runAlterJobV2() {
 
             LOG.info("backend {} lefts {} replicas to decommission: {}{}", beId, backendTabletIds.size(),
                     backendTabletIds.subList(0, Math.min(10, backendTabletIds.size())),
-                    hasWal ? "; and has unfinished WALs" : "");
+                    walNum > 0 ? "; and has " + walNum + " unfinished WALs" : "");
         }
     }
 
@@ -210,10 +210,6 @@ private boolean checkTablets(Long beId, List<Long> backendTabletIds) {
         return false;
     }
 
-    private boolean checkWal(Backend backend) {
-        return Env.getCurrentEnv().getGroupCommitManager().getAllWalQueueSize(backend) == 0;
-    }
-
     private List<Backend> checkDecommission(DecommissionBackendClause decommissionBackendClause)
             throws DdlException {
         if (decommissionBackendClause.getHostInfos().isEmpty()) {

fe/fe-core/src/main/java/org/apache/doris/load/GroupCommitManager.java

@@ -106,7 +106,7 @@ public void waitWalFinished(long tableId) {
     /**
      * Check the wal before the endTransactionId is finished or not.
      */
-    public boolean isPreviousWalFinished(long tableId, List<Long> aliveBeIds) {
+    private boolean isPreviousWalFinished(long tableId, List<Long> aliveBeIds) {
         boolean empty = true;
         for (int i = 0; i < aliveBeIds.size(); i++) {
             Backend backend = Env.getCurrentSystemInfo().getBackend(aliveBeIds.get(i));
@@ -137,7 +137,7 @@ public long getAllWalQueueSize(Backend backend) {
         return size;
     }
 
-    public long getWalQueueSize(Backend backend, PGetWalQueueSizeRequest request) {
+    private long getWalQueueSize(Backend backend, PGetWalQueueSizeRequest request) {
         PGetWalQueueSizeResponse response = null;
         long expireTime = System.currentTimeMillis() + Config.check_wal_queue_timeout_threshold;
         long size = 0;
@@ -376,7 +376,7 @@ public void updateLoadData(long tableId, long receiveData) {
         }
     }
 
-    public void updateLoadDataInternal(long tableId, long receiveData) {
+    private void updateLoadDataInternal(long tableId, long receiveData) {
         if (tableToPressureMap.containsKey(tableId)) {
             tableToPressureMap.get(tableId).add(receiveData);
             LOG.info("Update load data for table{}, receiveData {}, tablePressureMap {}", tableId, receiveData,