Fix unused variable warning in sctp_output.c

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

sys/netinet/sctp_output.c:9367:33: error: variable 'cnt_thru' set but not used [-Werror,-Wunused-but-set-variable]
        int no_fragmentflg, bundle_at, cnt_thru;
                                       ^

The 'cnt_thru' variable was in sctp_output.c when it was first added,
but appears to have been a debugging aid that has never been used, so
remove it.

MFC after: 3 days