Remove null checks for desks when removing tiled tasks.

Desks can be removed without transitions specially if they are not visible.

This change removes a null check for removing tiled tasks from non-existent
desks and gracefully continues execution.

Flag: com.android.window.flags.enable_tile_resizing
Test: EXEMPT, removing null check, tested on device.
Bug: 416139364
Change-Id: Ia73465db76aada067c0b84083e43418f6a863daa