Remove mNeedsLetterboxedAnimation

The field was added in I7c173c8711c9982f5cc66931d36d564abf923a80.
Since the only caller AppTransitionController is removed with
legacy transition, now the field is always false.

Bug: 365884835
Flag: EXEMPT remove outdated code
Test: atest AppCompatLetterboxUtilsTest LetterboxTest
            AppCompatLetterboxPolicyTest

Change-Id: I1be85060f13bc1ba31b48649988fe46e99e2c80c