Desks: Unset desk as active in display when removed

Fixes a bug where DesktopDisplay#activeDeskId could remain non-null and
referencing a desk that was already removed.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 389960283
Test: remove desk with adb command or from overview, dumpsys desktopmode
shows activeDeskId=null

Change-Id: I10bc578bdf0ffb8932f8426e896384181add5b92