dereg acl-rule counters during acl-table del (sonic-net#2574)

*Deleting ACL Tables deletes all the associated Rules but it doesn't delete the associated counters.

Author: vivekrnv

orchagent/aclorch.cpp

@@ -2588,6 +2588,12 @@ bool AclTable::clear()
     for (auto& rulepair: rules)
     {
         auto& rule = *rulepair.second;
+
+        if (rule.hasCounter())
+        {
+            m_pAclOrch->deregisterFlexCounter(rule);
+        }
+
         bool suc = rule.remove();
         if (!suc)
         {