Remove Yawning/chacha20 from vendoring

- Newer code may no longer be used due to
  license incompatibility

psiphon/common/prng/prng.go

@@ -59,8 +59,8 @@ import (
 	"time"
 
 	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common"
+	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/crypto/Yawning/chacha20"
 	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/crypto/hkdf"
-	"github.com/Yawning/chacha20"
 )
 
 const (

psiphon/common/quic/obfuscator.go

@@ -30,9 +30,9 @@ import (
 
 	"github.com/Psiphon-Labs/goarista/monotime"
 	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common"
+	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/crypto/Yawning/chacha20"
 	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/crypto/hkdf"
 	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/prng"
-	"github.com/Yawning/chacha20"
 )
 
 const (

vendor/vendor.json

@@ -80,12 +80,6 @@
 			"revision": "1f81de88145c342aad771f4f630012618faaffa7",
 			"revisionTime": "2018-04-25T19:07:11Z"
 		},
-		{
-			"checksumSHA1": "iZ3cpzMPHGeYQWcSLweeYdPqPiw=",
-			"path": "github.com/Yawning/chacha20",
-			"revision": "e3b1f968fc6397b51d963fee8ec8711a47bc0ce8",
-			"revisionTime": "2017-09-04T08:51:04Z"
-		},
 		{
 			"checksumSHA1": "30PBqj9BW03KCVqASvLg3bR+xYc=",
 			"path": "github.com/agl/ed25519/edwards25519",