Fix unused variable warning in if_alc.c

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

sys/dev/alc/if_alc.c:3441:6: error: variable 'prog' set but not used [-Werror,-Wunused-but-set-variable]
        int prog;
            ^

The 'prog' variable seems to be a left-over from some debugging code
that no longer exists, and can be removed without any functional change.

MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D35831