backend/local: fix "operation not supported" when unlocking

If the repo is on a mounted folder that doesn't support chmod (like
SMB), it was causing an "operation not supported" error when trying to
chmod 666 a file before deleting it.

But it isn't generally needed before deleting a file (the folder
permissions matter there, not the file permissions). So, just drop it.

changelog/unreleased/issue-5595

@@ -0,0 +1,8 @@
+Bugfix: Fix "chmod not supported" errors when unlocking
+
+Restic 0.18.0 introduced a bug that caused "chmod xxx: operation not supported"
+errors to appear when unlocking with a stale lock, on a local file repository
+that did not support chmod (like CIFS or WebDAV mounted via FUSE). Restic now
+just doesn't bother calling chmod in that case on Unix, as it is unnecessary.
+
+https://github.com/restic/restic/issues/5595

internal/backend/local/local.go

@@ -255,13 +255,7 @@ func (b *Local) Stat(_ context.Context, h backend.Handle) (backend.FileInfo, err
 func (b *Local) Remove(_ context.Context, h backend.Handle) error {
 	fn := b.Filename(h)
 
-	// reset read-only flag
-	err := os.Chmod(fn, 0666)
-	if err != nil && !os.IsPermission(err) {
-		return errors.WithStack(err)
-	}
-
-	return os.Remove(fn)
+	return removeFile(fn)
 }
 
 // List runs fn for each file in the backend which has the type t. When an

internal/backend/local/local_unix.go

@@ -52,3 +52,7 @@ func setFileReadonly(f string, mode os.FileMode) error {
 
 	return err
 }
+
+func removeFile(f string) error {
+	return os.Remove(f)
+}

internal/backend/local/local_windows.go

@@ -2,6 +2,8 @@ package local
 
 import (
 	"os"
+
+	"github.com/restic/restic/internal/errors"
 )
 
 // Can't explicitly flush directory changes on Windows.
@@ -16,3 +18,14 @@ func isMacENOTTY(_ error) bool { return false }
 func setFileReadonly(_ string, _ os.FileMode) error {
 	return nil
 }
+
+func removeFile(f string) error {
+	// Reset read-only flag,
+	// as Windows won't let you delete a read-only file
+	err := os.Chmod(f, 0666)
+	if err != nil && !os.IsPermission(err) {
+		 return errors.WithStack(err)
+	}
+
+	return os.Remove(f)
+}