omxvideoenc: remove GST_PARAM_MUTABLE_PLAYING from 'max-bitrate' property
omxvideoenc: remove GST_PARAM_MUTABLE_PLAYING from 'max-bitrate' property

This property isn't actually mutable in the PLAYING state.

https://bugzilla.gnome.org/show_bug.cgi?id=793458

GStreamer/gst-omx,GStreamer/gst-omx,GStreamer/gst-omx