Remove flag single_hop_screenshots

Flag has been rolled out for several months and is safe to
remove along with any legacy methods and dead code.

Bug: b/351894825
Test: atest SurfaceFlinger_test
Flag: EXEMPT flag removal
Change-Id: Id6fdfc534a0bff6ff9dfb8727c08ff821bc4965a
diff --git a/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h b/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
index 81b929b..6778af3 100644
--- a/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
+++ b/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
@@ -475,8 +475,7 @@
 
         return mFlinger->renderScreenImpl(renderArea.get(), buffer, regionSampling,
                                           false /* grayscale */, false /* isProtected */,
-                                          false /* attachGainmap */, captureResults, displayState,
-                                          layers);
+                                          captureResults, displayState, layers);
     }
 
     auto getLayerSnapshotsForScreenshotsFn(ui::LayerStack layerStack, uint32_t uid) {