Disable Playback Speed feature with chromecast

Description

It seems that it's not a good idea to modify the playback speed while streaming with Chromecast, so we should disable this feature like we do with the subtitles.

See #1648 (closed)

changed milestone to %3.4.x

added Important label

mentioned in merge request !987 (merged)

closed with merge request !987 (merged)

closed with commit ce05ea61