Use CWallet::SignTransaction in CreateTransaction and signrawtransactionwithwallet

Summary:
Instead of duplicating signing code, just use the function we already
have.

Backport of Core PR18115 part [4/9] : https://github.com/bitcoin/bitcoin/pull/18115/commits/a4af324d15c1ee43c2abd11a304ae18c7ee82eb0

Depends on D8098

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8100