I suppose you could try the below format function.
Code:
[cb:parse function="clean" method="string"]CONTENTS_HERE[/cb:parse]
That applies a string conversion. Not sure if it'll remove the linebreaks though. If it doesn't you may need to just use a Code action and use PHP to further process the activity message. There is a replace format function, but I doubt it'll replace linebreak characters, but you can give it a try with the below.
Code:
[cb:parse function="replace" search="\n" replace=""]CONTENTS_HERE[/cb:parse]