[CMAKE] Drop native build dependency on Openssl
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.
BCHN note: This is part of a clean backporting of changes submitted in !928 .
Differential Revision: https://reviews.bitcoinabc.org/D7816