Remove unneeded "finished" signal in QThread

[gmarmstrong]

Quoting @deeplow:

QThread already sends a "finished" signal when it ends. This also
solves a previously introduced bug where the signal had a signature
of one bool argument representing if there was an error or not.

This arg's inclusion was introduced in commit ea47a2e
but seems it was added by mistake as the slot that catches the
signal (ConvertWidget.all_done) doesn't have any arguments.

This last part in particular was introducing an error in Qt6
since the signal became a private signal, whereas Qt 5 had
defined it as a just a signal 1.