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Also, removes the code that supports them.

Affected exit codes:
  * USER_IMAGE_SELECTED
    * user image screen has been removed from (in
      session) OOBE, and will be added to discover screen, which means,
      wizard controller does not have to handle the screen exit code
      any longer
  * ENCRYPTION_MIGRATION_FINISHED
    ENCRYPTION_MIGRATION_SKIPPED
    * Encryption migration screen is not reporting result to wizard
      controller (instead it provides interface to add callbacks to be
      called upon completion)
  * CONNECTION_FAILED
    * Not used anywhere

While here, remove HIDDetectionScreen::Delegate, as HIDDetectionScreen
is not callign into the delegate anymore.

BUG=930267

Change-Id: I03d3b28270967032fa66db29e4a4ac9e2d64655f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1501092
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637943}