[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/interfaces/chain.cpp

@@ -339,7 +339,6 @@ public:
     CFeeRate relayMinFee() override { return ::minRelayTxFee; }
     CFeeRate relayIncrementalFee() override { return ::incrementalRelayFee; }
     CFeeRate relayDustFee() override { return ::dustRelayFee; }
-    CAmount maxTxFee() override { return ::maxTxFee; }
     bool getPruneMode() override { return ::fPruneMode; }
     bool p2pEnabled() override { return g_connman != nullptr; }
     bool isReadyToBroadcast() override { return !::fImporting && !::fReindex && !IsInitialBlockDownload(); }