[CMAKE] Cleanup secp256k1 module path

Summary:
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.

Test Plan:

ninja check check-secp256k1

ABC_BUILD_NAME=build-default ./contrib/teamcity/build-configurations.sh

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4846