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