Fix unused variable warning in if_lagg.c

With clang 15, the following -Werror warning is produced:

sys/net/if_lagg.c:2413:6: error: variable 'active_ports' set but not used [-Werror,-Wunused-but-set-variable]
        int active_ports = 0;
            ^

The 'active_ports' variable appears to have been a debugging aid that
has never been used for anything (ref https://reviews.freebsd.org/D549),
so remove it.

MFC after: 3 days