sweep+contractcourt: track best height in UtxoSweeper

Thus we can use shorter method signatures. In doing so we also remove an
old TODO in one use case of `CreateSweepTx`.

contractcourt/interfaces.go

@@ -53,8 +53,8 @@ type UtxoSweeper interface {
 	// CreateSweepTx accepts a list of inputs and signs and generates a txn
 	// that spends from them. This method also makes an accurate fee
 	// estimate before generating the required witnesses.
-	CreateSweepTx(inputs []input.Input, feePref sweep.FeePreference,
-		currentBlockHeight uint32) (*wire.MsgTx, error)
+	CreateSweepTx(inputs []input.Input,
+		feePref sweep.FeePreference) (*wire.MsgTx, error)
 
 	// RelayFeePerKW returns the minimum fee rate required for transactions
 	// to be relayed.