Clean up the legacy recent root task

910089a8, add the recents root task for legacy split screen.
System switched to use the stage split from android S and there is no
use case to reparent recent task to split-root. So that we should
clean up the legacy code.

Bug: 159029784
Test: atest WmTests:RootTaskTests
      - Using the 3p launcher.
      - Enter the split-screen and lunch the recents app
Change-Id: If67b90e5edaecfa2daa14cea9f610377b376799b