[User Switcher] Remove manage user in phone dialog

Removes the manage user icon in the user switcher dialog on the phone.
This is because the user settings button serves this function.

Bug: 254257455
Test: Turn off fullscreen user switcher flag with `$adb shell cmd statusbar flag 506 off` and then observe the manage user icon. Observe that it is gone after changes. Manage user remains in the bouncer and is not a visible item in full screen user switcher.
Change-Id: Id86f21dd0f182cfb81ec08e32a6a9240563272c4