[CMAKE] Drop native build dependency on Openssl

Summary:
By disabling BIP70 and the secp256k1 Openssl tests for the native build,
the dependency is gone.

Test Plan:

cmake -GNinja ..
ninja

Check in the cmake run for the native build that openssl is no longer
searched.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7816