[backport#16451 2/3][wallet] Flatten CWalletTx class hierarchy

Summary:
Removes CMerkleTx as a base class for CWalletTx. Serialization logic is
moved from CMerkleTx to CWalletTx.

https://github.com/bitcoin/bitcoin/pull/16451/commits/783a76f23ba4f33b6e6f609eaf3bf41afd9bcd6f

Depends on D7038

Partial backport of Core PR16451

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7039