security/botan2: Remove expired port
2025-03-31 security/botan2: End of life 2024-12-31 upstream, consider migrating to security/botan3
With keepass286 moved to botan3 and softhsm2 using OpenSSL by default,
we can strip softhsm2's botan2 non-default option and remove botan2 from
the tree.