Move to Opus only

[fridtjof]

Opus was chosen as the only codec going forward in Mumble 1.2.4, released in 2013: https://www.mumble.info/blog/mumble-1.2.4-released/

12 years later, it is safe to assume nobody is running Mumble clients that do not support Opus anymore. As #70 got rid of the dedicated CELT library already, let's throw Speex out as well.

Finally, we can then probably reduce the codec negotiation/choice logic a bit.

[Krzmbrzl]

FWIW the desktop client has removed support for everything but Opus in 1.5 as well. Though we actually got a report from a user that was seeing issues because of it. But that should be the absolute exception...

[fridtjof]

Hm, I see. Yeah, in the interest of giving the iOS stuff another chance to live on I think I'd rather get rid of any dependency where possible, given how terrible integrating them with XCode is :D

[Krzmbrzl]

Yeah absolutely. With my statement I wanted to give another reason for dropping support for anything but Opus :)