This is a follow-up for D144. It converts the wallet to use
ContextualCheckTransactionForCurrentBlock() instead of the
CheckFinalTx() wrapper and removes the unused finaltx.cpp/h files.

This is a preamble to backporting PR15288 which will remove the direct call to CheckFinalTx in the wallet (this commit).
Having both CheckFinalTx and ContextualCheckTransactionForCurrentBlock is error prone when doing the node/wallet separation backports. This cleanup make it easier to backport and review.
Note that the above commit will allow to remove some of the boilerplate introduced in this diff.