Fix unused variable warning in iflib.c

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

sys/net/iflib.c:993:8: 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

(cherry picked from commit 0294e95da4aa272906d4472137b1235d8d8a7180)