[Tech debt] Cleanup repo

[mratsim]

Part of the tech debt week https://github.com/status-im/nim-beacon-chain/issues/867

With the varying schemes we had we have accumulated some cruft in the repo. In preparation for auditing, the code paths that are not used should be cleaned up.

For example

• bls_old_spec: https://github.com/status-im/nim-blscurve/blob/9efb20df/blscurve/bls_old_spec.nim which was kept in case we needed to revert to 0.9.x BLS signature scheme
• hashToG2: https://github.com/status-im/nim-blscurve/blob/9efb20df/blscurve/common.nim#L799
• random for secret key generation: https://github.com/status-im/nim-blscurve/blob/9efb20df/blscurve/common.nim#L861

[mratsim]

Closed by #49

[mratsim]

Reopening

Another cleanup round: https://github.com/status-im/nim-blscurve/blob/b435f1a7296fceda8a68aaca391e6d76b2c632d3/blscurve/common.nim#L760-L801 was missed.

And when HashToCurve v7 lands and the v5 can be completely retired (probably soon as Schlesi testnet is down), we need to cleanup v5 code.

[mratsim]

Closed by #65