Delete stale hidden IPs in FIB.

Delete stale hidden IPs in FIB.

Issue:
------
When VM is deleted without port being deleted, learnt hidden IPs are
not removed from the adjacency list. When another VM with same port is
created, the learnt hidden IPs which existed in  the adjacency list
will result into programming these IPs in FIB.
This issue exists for both IPv4 and IPv6 addresses.

Fix:
----
Removed learnt hidden IPs from the adjacency list during
processVpnInterfaceDown().

JIRA: NETVIRT-1363

Change-Id: I606f49105c5eee5f76bbae908eb626d4ed346a23
Signed-off-by: Somashekar Byrappa <somashekar.b@altencalsoftlabs.com>