Revert qtmux: Use buffer PTS if DTS is not set"

This reverts commit f72c3cf71fde622067f41f31a53978ba4c94469d.

https://bugzilla.gnome.org/show_bug.cgi?id=707340

gst/isomp4/gstqtmux.c

@@ -2229,10 +2229,7 @@ gst_qt_mux_add_buffer (GstQTMux * qtmux, GstQTPad * pad, GstBuffer * buf)
     if (pad->have_dts) {
       gint64 scaled_dts;
       if (pad->last_buf) {
-        if (GST_BUFFER_DTS_IS_VALID (pad->last_buf))
-          pad->last_dts = GST_BUFFER_DTS (pad->last_buf);
-        else
-          pad->last_dts = GST_BUFFER_PTS (pad->last_buf);
+        pad->last_dts = GST_BUFFER_DTS (pad->last_buf);
       } else {
         pad->last_dts = GST_BUFFER_DTS (last_buf) +
             GST_BUFFER_DURATION (last_buf);