Removed onBackPressed handling in SplitSCreenSelectState

- So it'll have exact same behavior as OverviewState to exit overview when going back
- Also removed RecentsView.cancelSplitSelect as it's longer needed

Fix: 181707736
Test: manual
Change-Id: Ide4dfc64680ecc9adfe245ae2de1463735b0490e

quickstep/src/com/android/launcher3/uioverrides/states/SplitScreenSelectState.java

@@ -30,11 +30,6 @@ public class SplitScreenSelectState extends OverviewState {
         super(id);
     }
 
-    @Override
-    public void onBackPressed(Launcher launcher) {
-        launcher.getStateManager().goToState(OVERVIEW);
-    }
-
     @Override
     public int getVisibleElements(Launcher launcher) {
         return SPLIT_PLACHOLDER_VIEW;