multi: remove witness parameter from 'NewAltLeaf`

Since we're only ever passing nil (and we use the previous witness
in the script/burn key), we just remove the witness parameter
completely.

Author: gijswijs

asset/asset.go

@@ -1914,9 +1914,7 @@ type AltLeaf[T any] interface {
 }
 
 // NewAltLeaf instantiates a new valid AltLeaf.
-func NewAltLeaf(key ScriptKey, keyVersion ScriptVersion,
-	prevWitness []Witness) (*Asset, error) {
-
+func NewAltLeaf(key ScriptKey, keyVersion ScriptVersion) (*Asset, error) {
 	if key.PubKey == nil {
 		return nil, fmt.Errorf("script key must be non-nil")
 	}
@@ -1927,7 +1925,7 @@ func NewAltLeaf(key ScriptKey, keyVersion ScriptVersion,
 		Amount:              0,
 		LockTime:            0,
 		RelativeLockTime:    0,
-		PrevWitnesses:       prevWitness,
+		PrevWitnesses:       nil,
 		SplitCommitmentRoot: nil,
 		GroupKey:            nil,
 		ScriptKey:           key,

asset/mock.go

@@ -678,12 +678,9 @@ func RandAssetWithValues(t testing.TB, genesis Genesis, groupKey *GroupKey,
 
 // RandAltLeaf generates a random Asset that is a valid AltLeaf.
 func RandAltLeaf(t testing.TB) *Asset {
-	randWitness := []Witness{
-		{TxWitness: test.RandTxWitnesses(t)},
-	}
 	randKey := RandScriptKey(t)
 	randVersion := ScriptVersion(test.RandInt[uint16]())
-	randLeaf, err := NewAltLeaf(randKey, randVersion, randWitness)
+	randLeaf, err := NewAltLeaf(randKey, randVersion)
 	require.NoError(t, err)
 	require.NoError(t, randLeaf.ValidateAltLeaf())