Since commit e134c203 strdups of several elements of a manifest are kept
in the DASHContext; but said commit completely forgot to free these
strings again (with xmlFree()). Given that these strings are never used
at all, this commit closes this leak by reverting said commit.

This reverts commit e134c203.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>