Prevents showing Guests in switcher.

We already show "Guest" icon for adding a guest.
No need to show "Guest" users.

Guest users are ephemeral. They get cleaned up by the system.

Bug: 77491605
Test: On mojave, adding of many Guest sessions and observing the switcher.
Change-Id: Id3e10e01b7dbf0070605d09777a2fb3bedf585c0