Remove the various dead code from `src/allmydata/util/base32.py`

[exarkun]

• could_be_base32_encoded is "look before you leap". If you want to know if it is base32 encoded, decode it and find out. This should be removed.
• A lot of module-level constants like c2vtranstable and rfc3548_alphabet are unnecessary if we don't have could_be_base32_encoded.

Delete the stuff in this module that we don't actually use or need.