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#testCalculateBoundingRects_imeCaptionBar
  InsetsSourceTesttestCalculateBoundingRects_imeCaptionBarFrameMisaligned_rectsFixedToBottom
Change-Id: I79649705e3d44bc5224bde97050d1283019d23f7