hashing: Remove io.WriterTo implementation

This functionality has gone unused since
4b3dc415ef changed hashing.Reader's only
client to use ioutil.ReadAll on a bufio.Reader wrapping the hashing
Reader.

Reverts bcb852a8d0.

internal/hashing/reader_test.go

@@ -7,26 +7,8 @@ import (
 	"io"
 	"io/ioutil"
 	"testing"
-
-	rtest "github.com/restic/restic/internal/test"
 )
 
-// only expose Read method
-type onlyReader struct {
-	io.Reader
-}
-
-type traceWriterTo struct {
-	io.Reader
-	writerTo io.WriterTo
-	Traced   bool
-}
-
-func (r *traceWriterTo) WriteTo(w io.Writer) (n int64, err error) {
-	r.Traced = true
-	return r.writerTo.WriteTo(w)
-}
-
 func TestReader(t *testing.T) {
 	tests := []int{5, 23, 2<<18 + 23, 1 << 20}
 
@@ -39,44 +21,22 @@ func TestReader(t *testing.T) {
 
 		expectedHash := sha256.Sum256(data)
 
-		for _, test := range []struct {
-			innerWriteTo, outerWriteTo bool
-		}{{false, false}, {false, true}, {true, false}, {true, true}} {
-			// test both code paths in WriteTo
-			src := bytes.NewReader(data)
-			rawSrc := &traceWriterTo{Reader: src, writerTo: src}
-			innerSrc := io.Reader(rawSrc)
-			if !test.innerWriteTo {
-				innerSrc = &onlyReader{Reader: rawSrc}
-			}
+		rd := NewReader(bytes.NewReader(data), sha256.New())
+		n, err := io.Copy(ioutil.Discard, rd)
+		if err != nil {
+			t.Fatal(err)
+		}
 
-			rd := NewReader(innerSrc, sha256.New())
-			// test both Read and WriteTo
-			outerSrc := io.Reader(rd)
-			if !test.outerWriteTo {
-				outerSrc = &onlyReader{Reader: outerSrc}
-			}
+		if n != int64(size) {
+			t.Errorf("Reader: invalid number of bytes written: got %d, expected %d",
+				n, size)
+		}
 
-			n, err := io.Copy(ioutil.Discard, outerSrc)
-			if err != nil {
-				t.Fatal(err)
-			}
+		resultingHash := rd.Sum(nil)
 
-			if n != int64(size) {
-				t.Errorf("Reader: invalid number of bytes written: got %d, expected %d",
-					n, size)
-			}
-
-			resultingHash := rd.Sum(nil)
-
-			if !bytes.Equal(expectedHash[:], resultingHash) {
-				t.Errorf("Reader: hashes do not match: expected %02x, got %02x",
-					expectedHash, resultingHash)
-			}
-
-			rtest.Assert(t, rawSrc.Traced == (test.innerWriteTo && test.outerWriteTo),
-				"unexpected/missing writeTo call innerWriteTo %v outerWriteTo %v",
-				test.innerWriteTo, test.outerWriteTo)
+		if !bytes.Equal(expectedHash[:], resultingHash) {
+			t.Errorf("Reader: hashes do not match: expected %02x, got %02x",
+				expectedHash, resultingHash)
 		}
 	}
 }