Replace secure_... intrinsics with insecure TensorFlow equivalents.

Designed for use in tests, this function replaces tff.federated_secure_{sum, sum_bitwidth, modular_sum} usages with equivalent TensorFlow computations. The resulting computation can then be run on TFF runtimes which do not implement secure computation.

comp The computation to transform.

comp with secure intrinsics replaced with insecure TensorFlow equivalents.