crypto/cipher: fix duplicated arguments to bytes.Equal in test

Args were duplicated by a mistake. Found using static analysis tools.

Change-Id: I2f61e09844bc409b1f687d654767332d93dd39a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/164937Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/crypto/cipher/cfb_test.go

@@ -81,7 +81,7 @@ func TestCFBVectors(t *testing.T) {
 		plaintextCopy := make([]byte, len(ciphertext))
 		cfbdec.XORKeyStream(plaintextCopy, ciphertext)
 
-		if !bytes.Equal(plaintextCopy, plaintextCopy) {
+		if !bytes.Equal(plaintextCopy, plaintext) {
 			t.Errorf("#%d: wrong plaintext: got %x, expected %x", i, plaintextCopy, plaintext)
 		}
 	}