Remove obsolete md4c patch

This was added in 1591a95 on top of an
older version of md4c, but those casts now exist in upstream code,
which we upgraded to in c29a235

Pick-to: 6.6 6.5 6.2
Change-Id: Ia1a939ed69bcd52ac8afd91dbd574767b59032d6
Reviewed-by: Kai Köhne <kai.koehne@qt.io>
(cherry picked from commit bdfe05e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>