When you delete a user the URL no longer links to the username. So if a search engine had previously indexed that URL then it will get a 404 error.
Now Google is telling you that they faced many 404 errors for your mobile site due to this. So google says "If these pages exist on your desktop site, [bad thing]". As the pages DO NOT exist on your desktop site either [bad thing] should be ignored.
Furthermore, Google ends up saying "If these URLs don't exist, no action is necessary". So no action is necessary :)