backup: extract StdioWrapper from ProgressPrinters

The StdioWrapper is not used at all by the ProgressPrinters. It is
called a bit earlier than previously. However, as the password prompt
directly accessed stdin/stdout this doesn't cause problems.

internal/ui/backup/progress.go

@@ -2,7 +2,6 @@ package backup
 
 import (
 	"context"
-	"io"
 	"sync"
 	"time"
 
@@ -22,10 +21,6 @@ type ProgressPrinter interface {
 	Finish(snapshotID restic.ID, start time.Time, summary *Summary, dryRun bool)
 	Reset()
 
-	// ui.StdioWrapper
-	Stdout() io.WriteCloser
-	Stderr() io.WriteCloser
-
 	P(msg string, args ...interface{})
 	V(msg string, args ...interface{})
 }