Disable part files for OC ext storage backend + s2s backend

When uploading files to an OC ext storage backend or when using server
to server sharing storage, part files aren't needed because the backend
already has its own part files and takes care of the final atomic rename
operation.

This also fixes issues when using two encrypted ownCloud instances where
one mounts the other either as external storage (ownCloud backend) or
through server to server sharing.