[Optim] Replace EIP2333 routines by builtin ones

[mratsim]

BLST added builtin support for EIP2333 which is currently implemented manually in https://github.com/status-im/nim-blscurve/tree/master/blscurve/eth2_keygen

• https://github.com/supranational/blst/commit/4e1935eb722289789f5b8f8447415f77b63ff37c
• https://github.com/supranational/blst/commit/794b2b509d06e714c729a65fa4d379fa277eb0da

Note: those procedures aren't used in Nimbus at the moment as we redirect all keygen needs to the eth-deposit-cli

[mratsim]

closed by #142