Mark netconf related factory as functional interface

- deprecate/remove redundant factory implementations

Change-Id: I29ba9f3397c37c02b37d07dff16a4203186c5fcd
diff --git a/protocols/netconf/api/src/main/java/org/onosproject/netconf/NetconfSessionFactory.java b/protocols/netconf/api/src/main/java/org/onosproject/netconf/NetconfSessionFactory.java
index 0e5e9be..806eccb 100644
--- a/protocols/netconf/api/src/main/java/org/onosproject/netconf/NetconfSessionFactory.java
+++ b/protocols/netconf/api/src/main/java/org/onosproject/netconf/NetconfSessionFactory.java
@@ -19,6 +19,7 @@
 /**
  * Abstract interface for the creation of a NETCONF session.
  */
+@FunctionalInterface
 public interface NetconfSessionFactory {
 
     /**