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/commit_sweep_resolver_test.go

@@ -140,8 +140,8 @@ func (s *mockSweeper) SweepInput(input input.Input, params sweep.Params) (
 	return result, nil
 }
 
-func (s *mockSweeper) CreateSweepTx(inputs []input.Input, feePref sweep.FeePreference,
-	currentBlockHeight uint32) (*wire.MsgTx, error) {
+func (s *mockSweeper) CreateSweepTx(inputs []input.Input,
+	feePref sweep.FeePreference) (*wire.MsgTx, error) {
 
 	// We will wait for the test to supply the sweep tx to return.
 	sweepTx := <-s.createSweepTxChan