zebra: Move where zebra marks a nhg as uninstalled in fib

zebra: Move where zebra marks a nhg as uninstalled in fib

Currently the code is marking the nhg as uninstalled but not
causing that to flood up to the dependent nhgs:

nhg 3 is a group of 1/2
   1 -> interface A
   2 -> interface B

Suppose A goes down, old code would mark nhg 1 as !VALID and !INSTALLED.
Suppose B then goes down, old code would mark nhg 2 as !VALID and !INSTALLED
But would not mark nhg 3 as !VALID and !INSTALLED (sort of assuming that
it would just be cleaned up by NHG refcounts ).  I would prefer that
the code is pedantic about nhg 3 actually being removed from the system.

This code moves the setting of !INSTALLED into zebra_nhg.c where it
really belongs.

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