[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/node.h

@@ -159,9 +159,6 @@ public:
     //! Get network active.
     virtual bool getNetworkActive() = 0;
 
-    //! Get max tx fee.
-    virtual CAmount getMaxTxFee() = 0;
-
     //! Estimate smart fee.
     virtual CFeeRate estimateSmartFee(int num_blocks, bool conservative, int* returned_target = nullptr) = 0;