qtmux: Revert commits that set DTS and duration on buffers unconditionally

qtmux: Revert commits that set DTS and duration on buffers unconditionally

39f7e52266fde3b3c035e22cbcbb2bb1fa207b17 was setting the buffer duration
to 0 if is not valid, under the assumption that this is "the last"
buffer and no others are coming next. This is wrong, last_buf is the
previous buffer and not the very last one.

4e3c13c87c258c9c95e2217d32ab314d12b5fffc was setting DTS to 0 if there
was none. This will set DTS to 0 for all e.g. audio streams, completely
messing up calculations if streams don't start at 0.

https://bugzilla.gnome.org/show_bug.cgi?id=774840