remove the zk part in spartan_parallel

[kunxian-xia]

The zk property in the old spartan_parallel which use DLOG-based field is achieved by using a homomorphic commitment. This kind of primitive does not exist in small-field like goldilocks and its extension. Therefore, we plan to remove the support of zk as the next step after merging #17.

[darth-cy]

Resolve in PR: #53

[kunxian-xia]

This issue is resolved in #53.