[processing] Correctly delete processing dialogs

[arnaud-morvan]

Description

del dlg do not delete the QT dialog, only the python variable.

Relate with 5c844a5

Checklist

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[nyalldawson]

I think this is the 3rd time someone's tried to fix this issue ;) I wonder if we're better off just setting the Qt.WA_DeleteOnClose flag on the dialogs here?

[arnaud-morvan]

As sometimes we need to get dialog properties after it has been closed, this seems better to me, for consistency. If we can merge this, we will get rid of annoying popups when closing the project as layers are removed from the layer tree.