Remove CombineSignatures and replace tests

Summary:
Removes CombineSignatures and replaces its use in tests with
ProduceSignature to test the same behavior for ProduceSignature.

This is a backport of Core PR13425 : https://github.com/bitcoin/bitcoin/pull/13425/commits/b815600295bd5e7b274c8ee32da65f1419a79ab2

Depends on D4211

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4212