packet-a21.c: Minor changes

- Comment out several "set but unused" lines of code;
- initialize 'gcsna_handle' only once.

Change-Id: I17a89ccff228b28d075d9db9289ad412892b2639
Reviewed-on: https://code.wireshark.org/review/3837
Reviewed-by: Bill Meier <wmeier@newsguy.com>