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

Summary:
...

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

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This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/14424 | 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

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