[CMAKE] Cleanup secp256k1 module path
Now that secp256k1 has it's own project, there is no need to have it
build standalone from the bitcoin-abc source tree.
It can be selected by using the appropriated target instead.
ninja check check-secp256k1
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D4846