chore: remove FunctionInput::dummy (#4723)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

This function is unused.

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

acvm-repo/acir/src/circuit/opcodes/black_box_function_call.rs

@@ -10,12 +10,6 @@ pub struct FunctionInput {
     pub num_bits: u32,
 }
 
-impl FunctionInput {
-    pub fn dummy() -> Self {
-        Self { witness: Witness(0), num_bits: 0 }
-    }
-}
-
 #[derive(Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub enum BlackBoxFuncCall {
     AND {