Remove unused AES-128 code.
As far as I can tell this AES-128 code has never been in use in the project (outside of testing/benchmarking).
The AES-256 code is used in CCrypter::Encrypt/CCrypter::Decrypt (src/wallet/crypter.cpp).
Trivia: 0.15% of the project's C++ LOC count (excluding dependencies) is trimmed off:
$ LOC_BEFORE=$(git grep -I "" HEAD~1 -- "*.cpp" "*.h" ":(exclude)src/leveldb/" ":(exclude)src/secp256k1/" ":(exclude)src/univalue/" | wc -l)
$ LOC_AFTER=$(git grep -I "" -- "*.cpp" "*.h" ":(exclude)src/leveldb/" ":(exclude)src/secp256k1/" ":(exclude)src/univalue/" | wc -l)
$ bc <<< "scale=4; ${LOC_AFTER}/${LOC_BEFORE}"
.9985
:-)
