Remove witness option from nargo compile

[TomAFrench]

Problem

This flag seems quite out of place in my mind, due to how it's "skipping ahead" in the pipeline from code to generating a proof.

The analogy is that nargo execute is to nargo compile what cargo run is to cargo build and you would never pass runtime args to cargo build.

Solution

We can remove this flag and just use the nargo execute [witness_name] argument if we want to write a witness to file. This maintains the general flow of each command can only call into the previous steps to generate its inputs.

(There is an issue with my argument here with the nargo prove --verify flag also "skipping ahead" but I'd argue that this is acceptable as it's just a validity check rather than producing new outputs.)

Alternatives considered

Additional context