Change: Remove OnlyOffice extension

Tags: OnlyOffice

We've removed the OnlyOffice extension in oCIS. 
OnlyOffice has their own web extension for OC10 backend now with [a dedicated guide](https://owncloud.dev/clients/web/deployments/oc10-app/#onlyoffice).
In oCIS, we will follow up with a guide on how to start a WOPI server providing OnlyOffice soon.

https://github.com/owncloud/ocis/pull/2433
