Bugfix: Remove read-only files via the SFTP backend on Windows servers

Since restic 0.19.0, repository files on the SFTP backend are marked
read-only after save. On Windows SFTP servers, removing them failed
with a permission error. The SFTP backend now clears the read-only flag
before removing the file.

https://github.com/restic/restic/issues/21895
https://github.com/restic/restic/pull/21897
