Cleanup ENABLE_HIDE_IME_CAPTION_BAR flag

This flag has already been enabled for a while, and is included in
releases, so we can clean it up now.

Flag: EXEMPT cleanup
Bug: 375571655
Test: atest InputMethodServiceTest#testRequestHideImeCaptionBar
  InputMethodServiceTest#testRequestHideThenShowImeCaptionBar
  InsetsSourceTest#testCalculateInsets_imeCaptionBar
  InsetsSourceTest#testCalculateInsets_imeCaptionBar
  InsetsSourceTest#testCalculateBoundingRects_imeCaptionBar
  InsetsSourceTesttestCalculateBoundingRects_imeCaptionBarFrameMisaligned_rectsFixedToBottom
Change-Id: I809c69156c06ff5c06ca43ef4dd248f90d54f6d8
1 file changed