nexthops: Move code from remove_nexthop_from_groups to remove_nh_grp_entry

Move nh_grp dereference and check for removing nexthop group due to
all members gone into remove_nh_grp_entry.

Fixes: 430a0491 ("nexthop: Add support for nexthop groups")
Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>