Remove duplication of Flavor entity elements in Prover constructors

[zac-williamson]

In every Prover class (e.g. barretenberg/honk/proof_system/prover.cpp), in the constructor we copy every Flavor entity element from the proving key into the prover_polynomials member variable. The error surface of this step is large as one mis-copied member will break the prover and is difficult to spot + debug.

Ideally we can eithe remove or automate this copying step