Use a better and simpler default name for multi-file archives
Now the default name of a multi-file archive created from the file actions plugin (i.e. from Dolphin's context menu) will be "Archive.[extension]", not "[name of parent folder].[extension]". That was a weird default filename choice.
This partially reverts 77606102, which added a function that is no longer needed; that function has been removed.
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| cc @teams/vdg @fusionfuture |