qt: discard bookmark byte offset

This is useless given that the time offset is set and the input core
ignores the byte offset when the time offset is set. I think the column
should probably be removed completely.
input: remove dead and broken byte offset bookmark support (refs #8455)

Nobody would use this in real life (even time offset bookmarks are
hardly used at all), the "feature" was unreachable from the UI, and
it does not work with most demuxers.

seekpoint_t.i_byte_offset is kept due to other usage of the same
structure, i.e. proper seek points (as opposed to bookmarks).
input: delete stream from demux_Delete() (refs #8455)
input: remove input_source_t.p_stream (refs #8455)