I recently received a request from a client: can I prevent the milestones from showing up in the Portfolio Timeline view? The answer was: no, not natively anyway.
The trick is to hack the design of Asana by using a Chrome extension called Stylebot and load into it the modified design.
Comment below if you want to have access to the code!
Warning: by doing this you are purposefully modifying the design of the app. It means several things: if the design changes, the trick might not work anymore; you might not benefit from design improvements from Asana; please do not contact support for problems related to design before you tried deactivating that hack.