Remove default theme handling in Web

[JammingBen]

owncloud/ocis#9133 changed the theme loading in oCIS, which lead to the theme.json file moving from the Web repository to the oCIS repository.

We still have a theme.json file in Web though, which is currently being used as a fallback (if oCIS doesn't provide a theme or the theme is faulty?!). I don't think this makes much sense, especially since theme changes need to be applied in various places because of that. Ideally, oCIS is the single source of truth.