qtmux: Remove the pad from our internal list before calling collectpads

qtmux: Remove the pad from our internal list before calling collectpads

Previously we would end up with the collectpaddata structure already freed.
This would result in a bogus iteration of mux->sinkpads (all the
GstQTPad being freed) and it wouldn't be removed from that list.

Finally, due to it not being removed from that list, we would end up
calling a bogus gst_qt_mux_pad_reset on those structures => SEGFAULT