zebra: Uninstall NHG in some situations

zebra: Uninstall NHG in some situations

If you have this series of events:

a) Decision to install a NHG is made in zebra, enqueue to DPLANE
b) Changes to NHG are made and we remove it in the master pthread
   Since this NHG is not marked as installed it is not removed
   but the NHG data structure is deleted
c) DPLANE installs the NHG

In the end the NHG stays installed but ZEBRA has lost track of it.

Modify the removal code to check to see if the NHG is queued.
There are 2 cases:

a) NHG is kept around for a bit before being deleted.  In this case
just see that the NHG is Queued and keep it around too.

b) NHG is not kept around and we are just removing it.  In this case
check to see if it is queued and send another deletion event.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>