Remove obsolete OWA manifest language

[raucao]

The storage-first section of the spec currently contains language about using OWA manifests for RS app dashboards: https://tools.ietf.org/html/draft-dejong-remotestorage-06#section-11

Open Web App manifests have been obsoleted, and there's a new W3C spec in development that is just called Web App Manifest: https://w3c.github.io/manifest/

One can extend that manifest with custom properties, as long as the rules for naming are observed. But I'm not sure we actually want this feature in the final spec at all, because I have never seen a single implementation of it.