circom-prover: simplify the rapidsnark dependencies

[vivianjeng]

Problem

Now the circom-prover depends on ark-works zkey, and proving key structure
so even when using witnesscalc and rapidsnark, it converts from json string to ark-works types
and it depends on ark-works crates
e.g.

mopro/circom-prover/src/prover/rapidsnark.rs

Line 112 in b2446b3

let proving_key = read_proving_key::<_, Bn254>(&mut reader)?;

Details

1. Should we convert to string type like witnesscalc+rapidsnark?
2. or still keep it like
   

   mopro/circom-prover/Cargo.toml

   Lines 29 to 38 in b2446b3

   	rapidsnark = [
   	"rust-rapidsnark",
   	"serde_json",
   	"ark-bn254",
   	"ark-ec",
   	"ark-groth16",
   	"ark-bls12-381", # TODO: remove this
   	"ark-relations",
   	"ark-poly",
   	]