chore: fix warning in generated code

[sander2]

Without this change, the generated code is creating a lot of warning like these on nightly rust:

warning: type alias `UnderlyingSolTuple` is never used
    --> crates/bindings/src/utilities.rs:2643:18
     |
2643 |             type UnderlyingSolTuple<'a> = ();
     |                  ^^^^^^^^^^^^^^^^^^

[mattsse]

makes sense

[mattsse]

I see we only use this in the test below