MXS-1376 All zombie related code removed

As dcbs are now closed when dcb_close() is called and there is
no zombie queue, the zombie state can also be removed.

server/modules/protocol/MySQL/mysql_common.c

@@ -196,7 +196,7 @@ int mysql_send_com_quit(DCB*   dcb,
     CHK_DCB(dcb);
     ss_dassert(packet_number <= 255);
 
-    if (dcb == NULL || dcb->state == DCB_STATE_ZOMBIE)
+    if (dcb == NULL)
     {
         return 0;
     }