Remove add_gates_to_ensure_all_polys_are_non_zero()

[ledwards2225]

In the circuit builders we add dummy gates to ensure that all polynomials have at least one non-zero element in order to avoid the point-at-infinity commitment. We're getting closer to the point where we don't need this (for instance, at the time of writing, all Ultra/Mega honk tests pass without this. PG tests fail as do some recursion tests).

Aside from being an icky hack, its desirable to support circuits that don't use all of the functionality of the arithmetization. For example, right now if we removed the dummy gates folding verification for a circuit that performs no lookups would fail due to point at infinity commitments.