Fix unused variable warning in if_re_netmap.h

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

sys/dev/netmap/if_re_netmap.h:179:8: error: variable 'n' set but not used [-Werror,-Wunused-but-set-variable]
        u_int n;
              ^

The 'n' variable appears to have been a debugging aid that has never
been used for anything, so remove it.

MFC after: 3 days