Join split example cleanup

[codygunton]

Background

The join-split circuit example

barretenberg/cpp/src/barretenberg/join_split_example/proofs/join_split/join_split_circuit.cpp

Line 272 in a9fbc39

void join_split_circuit(Composer& composer, join_split_tx const& tx)

was stripped out of the old Aztec Connect repo and brought into Barretenberg as a demonstration of how a our standard library could be used in production. The tests have moreover been useful for benchmarking and for catching some subtle circuit changes, such as a change to ECDSA that was introduced to support the Noir backend.

Task

The code has come along for the ride through lots of refactoring. It could use a quick revamp. In particular, the parts involving mocking are a bit opaque and may be unnecessary.

[codygunton]

Closed by AztecProtocol/aztec-packages#5016