avformat/dashdec: Remove dead code

avformat/dashdec: Remove dead code

The code in question seems to have been copied from about 70 lines
above; yet the code here is only executed if some of the variables
(namely representation_segmenttemplate_node and fragment_template_node)
are NULL, so it makes no sense to check them for a child element.

Also remove a redundant resetting of a pointer to an AVFormatContext
after avformat_close_input() (which already sets the pointer to NULL).

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