backend: consistently use os package for filesystem access

The go std library should be good enough to manage the files in the
backend and cache folders.

internal/backend/local/local_unix.go

@@ -8,8 +8,6 @@ import (
 	"os"
 	"runtime"
 	"syscall"
-
-	"github.com/restic/restic/internal/fs"
 )
 
 // fsyncDir flushes changes to the directory dir.
@@ -45,5 +43,5 @@ func isMacENOTTY(err error) bool {
 
 // set file to readonly
 func setFileReadonly(f string, mode os.FileMode) error {
-	return fs.Chmod(f, mode&^0222)
+	return os.Chmod(f, mode&^0222)
 }