In `parse_chain_output_probe()` the corresponding input stream might receive EOS
and thus be removed before the actual pad is removed. So we cannot assert about
this in `parsebin_pad_removed_cb()`.
Also, driving-by, protect `find_input_stream_for_pad()` with the selection lock
similarly to other functions accessing the input streams list.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5887>
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