[backport#14424] Stop requiring imported pubkey to sign non-PKH schemes

Summary:
...

This is a fix for https://github.com/bitcoin/bitcoin/issues/14415

This is a backport of Core PR14424

Test Plan:

cmake .. -GENABLE_WERROR=ON -GNinja
ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6028