zebra: Remove unnecessary condition check for kernel routes

zebra: Remove unnecessary condition check for kernel routes

There are relaxed nexthop requirements for kernel routes because we
trust kernel routes.

Two minor changes for kernel routes:

1. `if_is_up()` is one of the necessary conditions for `if_is_operative()`.
Here, we can remove this unnecessary check for clarity.

2. Since `nexthop_active()` doesn't distinguish whether it is kernel route,
modified the corresponding comment in it.

Signed-off-by: anlan_cs <vic.lan@pica8.com>
(cherry picked from commit caf896d6ef18d917a9b9cee9cb48719ea9757e37)