Redundant degree check on commit key/commit

[vlopes11]

Why is it safe to use the default?

Originally posted by @moCello in #700 (comment)

[ashWhiteHat]

It returns errors in two cases.

1. polynomial coefficients are all zero
2. polynomial degree is higher than public parameters length

https://github.com/dusk-network/plonk/blob/master/src/commitment_scheme/kzg10/key.rs#L160

In my opinion.

Regarding case 1, the commitment for zero polynomial should be the identity of the curve point instead of error.
Regarding case 2, we can't commit a polynomial whose degree is higher than public parameters and should return an error instead of a return default.