Remove CheckFinalTx

Summary:
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.

Test Plan:

ninja all check check-functional

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5804