In the interface it's a separate command ("Break link to style"), but it seems in a script you have to use
Paragraph.applyParagraphStyle (using:ParagraphStyle[, clearingOverrides:bool=true])
You cannot use "null" or something similar for "ParagraphStyle", so I guess the idea is to use paragraph style #0 in your document (which is always "[No Paragraph Style]"). Be sure to set "clearing Overrides" to false, as it defaults to true.
The following single line works on your current paragraph:
app.activeDocument.selection.paragraphs.applyParagraphStyle (app.activeDocument.paragraphStyles, false);