[backport#16451 3/3][wallet] Remove CMerkleTx serialization logic

Summary:
CMerkleTx is only used for deserialization of old wallet files. Remove
the serialization logic, and tidy up CWalletTx serialization logic.

https://github.com/bitcoin/bitcoin/pull/16451/commits/05b56d1c937b7667ad51400d2f9fb674af72953f

Depends on D7039

Concludes 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/D7040