SF: Remove unused SurfaceFlinger::getLayerSortedByZForHwcDisplay

There were no callers, and it relied on CompositionEngine state being
made internal to the new code.

Test: atest libsurfaceflinger_unittest libcompositionengine_test
Bug: 121291683
Change-Id: I896ccc65c13acf3d9ea87c65da388b06f35ac175