Compatibility with Opencast plugin V3

[tgloeggl]

The new Opencast plugin in version 3 has now a different name and is not found anymore by the meetings plugin.

[tgloeggl]

fixed for studip_5x in 8c2aac3

[tibroc]

This seems like an error:
https://github.com/elan-ev/studip-meeting/blob/studip_5x/MeetingPlugin.php#L292

Why was this line introduced? It seems to break the logic connected to the cid-variable.