Remove computeImeLayeringTargetIfNeeded

This method was previously called from
ActivityRecord#onAnimationFinished, as part of the legacy transitions.
This caller was recently removed in [1], leaving this method unused.

  [1]: I5742d5e257b2a0edb1dd9ec7e2ca497dec94bf34

Flag: EXEMPT cleanup
Bug: 281029564
Test: n/a
Change-Id: Ie1ef739290917508ff0aca86291b8c7b4af1524c