decoder: don't set channels and sample rate in dictionary

This is redundant, the information can already be found in the AVStream
object and is set by avcodec_parameters_to_context. Let FFmpeg take care
of these details.

Change-Id: I199de87a852caeef34047e4f906ccd21dcb696c4
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>