zebra: remove NHG unhashable flag and its code

zebra: remove NHG unhashable flag and its code

Remove the code for setting a NHG as unhashable. Originally
this was to prevent us from attempting to put duplicates from
the kernel in our hashtable.

Now I think its better to not use them in the hashtable at all
and only track them in the ID table. Routes will still be able
to use them if they specify the ID explicitly when sending Zebra
the route, but 'normal' routes we hash the nexthop group on
will not.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>