internal/runtime/cgroup: remove duplicate readString definition

Both CL 723241 and CL 723581 added identical definitions.

Change-Id: I6a6a636c9e5f8c9080b9389ebf9d3f10305d79ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/725661
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: prattmic

src/internal/runtime/cgroup/line_reader_test.go

@@ -6,7 +6,6 @@ package cgroup_test
 
 import (
 	"internal/runtime/cgroup"
-	"io"
 	"strings"
 	"testing"
 )
@@ -122,18 +121,6 @@ var readerTests = []struct {
 	},
 }
 
-func readString(contents string) func(fd int, b []byte) (int, uintptr) {
-	r := strings.NewReader(contents)
-	return func(fd int, b []byte) (int, uintptr) {
-		n, err := r.Read(b)
-		if err != nil && err != io.EOF {
-			const dummyErrno = 42
-			return n, dummyErrno
-		}
-		return n, 0
-	}
-}
-
 func TestLineReader(t *testing.T) {
 	for _, tc := range readerTests {
 		t.Run(tc.name, func(t *testing.T) {