Stale flows in ELAN tables

Stale flows in ELAN tables

removeElanInterface method was getting called
after interface removal (from ElanInterfaceStateChangeListener)
and elan instance removal (from ElanInstanceManager).

So there was a case when elan instance removal gets called
first, wherein it won't delete the elan flows because interfaceInfo
(interface is deleted from interface config DS) is null.

The fix is to avoid not calling removeElanInterface when
elan instance remove event comes since openstack doesn't
allow deletion of network unless all ports are deleted.

Change-Id: Id80b779316b1f9fe3cf72145fc8991d590ec9d97
Signed-off-by: eswanit <swati.udhavrao.niture@ericsson.com>