HBASE-23153 PrimaryRegionCountSkewCostFunction SLB function should implement CostFunction#isNeeded (#714)

Signed-off-by: Reid Chan <reidchan@apache.org>

hbase-server/src/main/java/org/apache/hadoop/hbase/master/balancer/StochasticLoadBalancer.java

@@ -1236,6 +1236,11 @@ public class StochasticLoadBalancer extends BaseLoadBalancer {
         DEFAULT_PRIMARY_REGION_COUNT_SKEW_COST));
     }
 
+    @Override
+    boolean isNeeded() {
+      return cluster.hasRegionReplicas;
+    }
+
     @Override
     protected double cost() {
       if (!cluster.hasRegionReplicas) {