Delete port alias from portList set in portmgr. So that when the port…

… is re-created we can identify it as new configuration and set the default MTU. (sonic-net#1418)

Co-authored-by: Vasant Patil <vapatil@linkedin.com>

cfgmgr/portmgr.cpp

@@ -155,6 +155,7 @@ void PortMgr::doTask(Consumer &consumer)
         {
             SWSS_LOG_NOTICE("Delete Port: %s", alias.c_str());
             m_appPortTable.del(alias);
+            m_portList.erase(alias);
         }
 
         it = consumer.m_toSync.erase(it);