wire: remove erroneous witness size check in wire parsing

In this commit, we fix a bug that would cause nodes to be unable to
parse a given block from the wire. The block would be properly accepted
if fed in via other mechanisms.

The issue here is that the old checks for the maximum witness size,
circa segwit v0 where placed in the wire package _as well_ as the tx
engine. This check should only be in the engine, since it's properly
gated by other related scrip validation flags.

The fix itself is simple: limit witnesses only based on the maximum
block size in bytes, or ~4MB.

wire/msgtx.go

@@ -103,10 +103,9 @@ const (
 	maxWitnessItemsPerInput = 500000
 
 	// maxWitnessItemSize is the maximum allowed size for an item within
-	// an input's witness data. This number is derived from the fact that
+	// an input's witness data. This value is bounded by the largest
-	// for script validation, each pushed item onto the stack must be less
+	// possible block size, post segwit v1 (taproot).
-	// than 10k bytes.
+	maxWitnessItemSize = 4_000_000
-	maxWitnessItemSize = 11000
 )
 
 // TxFlagMarker is the first byte of the FLAG field in a bitcoin tx
@@ -588,8 +587,9 @@ func (msg *MsgTx) BtcDecode(r io.Reader, pver uint32, enc MessageEncoding) error
 			// item itself.
 			txin.Witness = make([][]byte, witCount)
 			for j := uint64(0); j < witCount; j++ {
-				txin.Witness[j], err = readScript(r, pver,
+				txin.Witness[j], err = readScript(
-					maxWitnessItemSize, "script witness item")
+					r, pver, maxWitnessItemSize, "script witness item",
+				)
 				if err != nil {
 					returnScriptBuffers()
 					return err