Remove ownCloud reference from the list of supported backend for files external

[oparoz]

This page should be fixed to remove all references to ownCloud.
https://github.com/nextcloud/server/blob/master/apps/files_external/lib/Lib/Backend/OwnCloud.php

Unless we need it to connect to very old instances, in which case there should maybe be some additional info such as "(legacy)"

[LukasReschke]

#906 will fix that

[MorrisJobke]

Merged and added to the backport -> closing