Remove the obsolete flat_witness feature flag from Nargo

[Savio-Sou]

Problem

The flattened witness format has been adopted as the default approach to formatting witnesses generated.

The flat_witness feature flag seems obsolete in consideration of so.

Happy Case

Merge-to-master or remove experimental works under the feature flag, and remove the flag.

Alternatives Considered

No response

Additional Context

No response

Would you like to submit a PR for this Issue?

No

Support Needs

No response

[kevaundray]

@vezenovm assigned this to you, please check if we do need this and document what has changed for us to no longer need this.

cc @TomAFrench