Remove tasks from repo on ExitDesktop transitions.

Another case of tasks not being removed from repo is when a task is
removed from desktop using an exit transition. We should also remove it
from desktop here.

Bug: 373932135
Test: atest DesktopTasskTransitionObserverTest
Flag: EXEMPT Bugfix
Change-Id: I4e6bc98a975c1390903b7cd90fa08c4ab640f1c4