pbrd: nhgc state improvements when installed/removed

pbrd: nhgc state improvements when installed/removed

Cleanup the marking of a nhc as installed/removed based on table
route installation.

We were not even handling the removal state at all.

We saw some timing issues with the routes being installed/removed
multiple times and then never resending the pbr map due to bad states
on the nhgc.

Dont worry about checking if its already marked installed before
scheduling the policy walk. We have a check in `pbr_send_map()`
to ensure we dont try to resend a map sequence already installed.

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