Remove OpenMina forks of num-bigint and num-rational

[dannywillems]

• remove fork openmina/num-bigint (https://github.com/openmina/openmina/blob/develop/Cargo.toml#L100). And use the same version as proof-systems. If any important commit is added in the fork, submit a patch upstream.

• remove fork openmina/num-rational (https://github.com/openmina/openmina/blob/develop/Cargo.toml#L101). If any important commit is added in the fork, submit a patch upstream.

Initially, it was in #1153.