Remove StatusBarState.FULLSCREEN_USER_SWITCHER

Was only used for downstream projects, and has since been removed from
those projects. There is an existing config:
config_enableFullscreenUserSwitcher which will be repurposed for
tablet user switching.

Bug: 217365397
Test: atest StatusBarTest KeyguardMediaControllerTest LockscreenShadeTransitionControllerTest StatusBarStateControllerImplTest StatusBarStateEventTest StatusBarKeyguardViewManagerTest
Change-Id: Idec504b682a244f4dde0c4ffcd50989365f29101