Remove log for desk root changes

Logger currently only filters for freeform changes and desk root changes
are freeform. Remove desk root change from logging so
desktop_mode_visible_tasks doesn't report desk root as a visible
freeform task.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Fix: 422076046
Test: manual logging
      atest DesktopModeLoggerTransitionObserverTest
Change-Id: If5f4bca42774a7e867daed0942c896af32419976