Remove buffer drop counter when port is removed (sonic-net#1860)

- What I did
I added the removal of port buffer drop counter on the deinit port flow.
it was added to the deInitPort function.

- Why I did it
the buffer drop counter was never removed when port was deleted.
this flex counter was create on the initPort function but it was removed when the port was deleted

- How I verified it
removed and created a port and check the FLEX_COUNTER redis table

FLEX_COUNTER_TABLE:PORT_BUFFER_DROP_STAT table

orchagent/portsorch.cpp

@@ -2423,6 +2423,13 @@ void PortsOrch::deInitPort(string alias, sai_object_id_t port_id)
     {
         port_stat_manager.clearCounterIdList(p.m_port_id);
     }
+
+    if (flex_counters_orch->getPortBufferDropCountersState())
+    {
+        port_buffer_drop_stat_manager.clearCounterIdList(p.m_port_id);
+    }
+
+
     /* remove port name map from counter table */
     m_counter_db->hdel(COUNTERS_PORT_NAME_MAP, alias);