[opt](stats) Scale replica of stats table to 3 when it's possible (#22227)

So that we could improve the availability of stats.

fe/fe-common/src/main/java/org/apache/doris/common/Config.java

@@ -1675,13 +1675,6 @@ public class Config extends ConfigBase {
     @ConfField
     public static int statistics_simultaneously_running_task_num = 10;
 
-    /**
-     * Internal table replica num, once set, user should promise the avaible BE is greater than this value,
-     * otherwise the statistics related internal table creation would be failed.
-     */
-    @ConfField
-    public static int statistic_internal_table_replica_num = 1;
-
     /**
      * if table has too many replicas, Fe occur oom when schema change.
      * 10W replicas is a reasonable value for testing.