[backport#16451 1/3][wallet] Move CMerkleTx functions into CWalletTx

Summary:
CMerkleTx only exists as a base class for CWalletTx and for wallet file
serialization/deserialization. Move CMerkleTx methods into CWalletTx,
but leave class hierarchy and serialization logic in place.

https://github.com/bitcoin/bitcoin/pull/16451/commits/b3a9d179f23f654b8fba63924bbca5fd31ad4bb0

Partial backport of Core PR16451

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7038