Removes the warning log

The log is no longer needed as it is a valid use case to
have direct child activities on a non-leaf TaskFragment.
An example is like the Activity#2 in below case.

Task
   - Activity#2
   - TaskFragment
      - Activity#1

Bug: 163976519
Test: wm presubmit
Change-Id: Ief62751802bb329e838dd2f4943afb9c38906a5b