zebra: stop deregistering static nexthops unless removing the static

zebra: stop deregistering static nexthops unless removing the static

Problem reported was that with some overlapping static route configurations,
when the link went down the less specific static was not re-installed after
the link came back up.  Determined that with the overlapping statics, we
would recursively resolve the next-hop temporarily thru the more specific
static route, but since the next-hop wasn't actually reachable, we would go
through the code that clears the nht information for the static completely.
This caused the nht code to no longer process the static route.

After reviewing the process, there doesn't seem to be any reason that the
static should be deregistered in that section of code.  Removed the
deregister and the problem is resolved and not addional failures seen in
manual testing.  zebra_test.py completed successfully and ospf and bgp smokes
completed with no new failures.

Ticket: CM-14873
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>
Reviewed-by: CCR-5696