[backport#16322] wallet: Fix -maxtxfee check by moving it to CWallet::CreateTransaction

Summary:
wallet: Remove unreachable code in CreateTransaction
test: Add test for maxtxfee option

Backport of Core PR16322

Test Plan:

ninja all check
./test/functional/test_runner.py wallet_create_tx

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6664