Thanks to this great topic, I've found the
FrontEndToken that I was looking for:
"DeleteGeneratedCells" (well, it is also written in
Help but I've found it later :)
I would love to execute it in
NotebookObject without confirmation through the automatically displayed
ChoiceDialog["Do You really want to..."].
Is it maybe a case of
I suspect I can deal with it by setting some global options for Mathematica but I would rather leave them.