avformat/dashdec: drop arbitrary DASH manifest size limit

avformat/dashdec: drop arbitrary DASH manifest size limit

Currently the utilized AVBPrint API is internally limited to unsigned
integers, so if we limit the file size as well as the amount to read
to UINT_MAX - 1, we do not require additional limiting to be performed
on the values.

This change is based on the fact that initially the 8*1024 value added
in 96d70694aea64616c68db8be306c159c73fb3980 was only for the case where
the file size was not known. It was not a maximum file size limit.

In 29121188983932f79aef8501652630d322a9974c this was reworked to be
a maximum manifest file size limit, while its commit message appears
to only note that it added support for larger manifest file sizes.

This should enable various unfortunately large MPEG-DASH manifests,
such as Youtube's multi-megabyte live stream archives to load up
as well as bring back the original intent of the logic.