Remove gamma from the groth16 snark

[imeckler]

This PR removes gamma from the Groth16 implementation (or put another way, it sets gamma equal to one). The benefit is that it makes the verification key one G2 element smaller, which is a big win from the point of view of proof composition.

From conversations with @arielgabizon, Mary Maller, and independent verification from @vanishreerao, I learned that this term is not actually necessary for any part of the security proof.

[dtebbs]

In r1cs_gg_ppzksnark_generator, don't you need to set G2_gen to one? Otherwise this value needs to appear in the resulting verification key.