Remove unnecessary layer assignment on destroy client surface

Because:
- Client surface doesn't have a computed layer value.
- If destroySurface removes a window, the removal
  postWindowRemoveCleanupLocked also updates layers.
- updateLetterboxSurfaceIfNeeded is no-op if the argument is null.

This mainly reduces unnecessary operations from
 activityStopped > destroySurfaces.

History commit: e12aece4

Bug: 163976519
Flag: EXEMPT remove outdate code
Test: CtsWindowManagerDeviceActivity
Change-Id: Ib11916f900eb8d30a7f9f6fdfc155420bbda34f6