[wallet] Move maxTxFee to wallet

This commit moves the maxtxfee setting to the wallet. There is only
one minor behavior change:

- an error message in feebumper now refers to -maxtxfee instead of
maxTxFee.

src/validation.cpp

@@ -252,7 +252,6 @@ uint256 hashAssumeValid;
 arith_uint256 nMinimumChainWork;
 
 CFeeRate minRelayTxFee = CFeeRate(DEFAULT_MIN_RELAY_TX_FEE);
-CAmount maxTxFee = DEFAULT_TRANSACTION_MAXFEE;
 
 CBlockPolicyEstimator feeEstimator;
 CTxMemPool mempool(&feeEstimator);