How not to show Universal Access menu Icon in task bar in Ubuntu 18.04?

Edit: as of Ubuntu 19.10, the "Remove Accessibility" extension is not anymore included in the Ubuntu Software Center. Despite not being updated for some time, the extension can be installed from the Gnome Extensions website and also works in 19.10. See second answer.

Open Ubuntu Software Center, click search sign at the right upper corner of the window and type in the search field "Gnome shell extensions Remove Accessibility" phrase. You'll see the "Remove Accessibility" Gnome shell extension. Install the extension and "Universal Access" icon has to disappear.

You can use a dedicated GNOME extension for this: Remove Accessibility