tapsend+asset: validate with altleaves from proof

Instead of using alt leaves from the inputs, we use the alt leaves from
the proof. We also remove the deduplication func
`AddLeafKeysVerifyMerge` because it's not needed anymore.

Author: gijswijs

tapsend/send.go

@@ -2009,6 +2009,19 @@ func ValidateAnchorInputs(anchorPacket *psbt.Packet, packets []*tappsbt.VPacket,
 			inputSiblings[outpoint] = sibling
 			inputScripts[outpoint] = anchorIn.WitnessUtxo.PkScript
 			inputAnchors[outpoint] = vIn.Anchor
+
+			// AltLeaves are the same for all assets from the same
+			// outpoint. So we only need to store them once for each
+			// outpoint.
+			if vIn.Proof == nil {
+				continue
+			}
+
+			if _, ok := inputAltLeaves[outpoint]; !ok {
+				inputAltLeaves[outpoint] = asset.CopyAltLeaves(
+					vIn.Proof.AltLeaves,
+				)
+			}
 		}
 	}