Delete BLS12Passthrough.sol since it's not supported on L2

[arthurgousset]

Scope

Delete BLS12Passthrough.sol. I can't see it being used anywhere in the monorepo, so we'll make necessary changes to CI in the Celo geth repo.

Context

From Slack:

Quick question: Any reason these two contracts should continue to exist as standalone contracts?
protocol/contracts/identity/test/ > RandomTest.sol. Are we keeping Random.sol on the L2?
protocol/contracts/common/libraries/test/ > BLS12Passthrough.sol. Are we keeping B12.sol on the L2?
I'm asking because RandomTest.sol and BLS12Passthrough.sol are the only smart contract tests that fail on forge test.
We can simply exclude them globally when running forge test, but I realised that I don't know if they are an artifact of the past or really in still use.
cc @blockchain-team (I believe RandomTest.sol is tested in the Cel1 geth client according to this PR: Add solidity wrappers for CIP-20/30/31 precompiles#10240)

BLS12Passthrough.sol is related to CIP-30/31, right? Those CIPs are not supported on the L2.