avformat/dashdec: Remove redundant checks

avformat/dashdec: Remove redundant checks

This commit removes two always-true checks as well as a dead default
case of a switch. The check when parsing manifests is always true,
because we now jump to the cleaning code in case the format of the
representation is unknown. The default case of the switch is dead,
because the type of the representation is already checked at the
beginning of parse_manifest_representation(). The check when reading
the header is dead, because we error out if an error happened before.

Reviewed-by: Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>