Remove `mockSignature()` in Semaphore Validator

[jimmychu0807]

Currently the validator check for mocksignature() because on the client side, we need to first call prepareUserOperation() to get a UserOp object for further process. During the call to prepareUserOperation(), it still need a valid signature as the bundler would simulate the transaction and verify that it passes. A valid signature need to be passed here, so a "mock signature" hole is added for this operation to pass. This need to be addressed in production.