quic: remove stray debugging print

For golang/go#58547

Change-Id: Ieca0569b23972d581e3bfc828c28eb2929f62c25
Reviewed-on: https://go-review.googlesource.com/c/net/+/504616
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

internal/quic/packet_protection.go

@@ -215,7 +215,6 @@ func (k keys) protect(hdr, pay []byte, pnumOff int, pnum packetNumber) []byte {
 // the unprotected payload and packet number.
 func (k keys) unprotect(pkt []byte, pnumOff int, pnumMax packetNumber) (pay []byte, num packetNumber, err error) {
 	if len(pkt) < pnumOff+4+headerProtectionSampleSize {
-		fmt.Println("too short")
 		return nil, 0, errInvalidPacket
 	}
 	numpay := pkt[pnumOff:]