[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/wallet.h

@@ -247,6 +247,9 @@ public:
     // Get default change type.
     virtual OutputType getDefaultChangeType() = 0;
 
+    //! Get max tx fee.
+    virtual CAmount getDefaultMaxTxFee() = 0;
+
     // Remove wallet.
     virtual void remove() = 0;