Revert "[a11y] Adds callback to notify screen reader when TTS engine connects."

This reverts commit 9e7e8382f1a052caace6c38ba7546685b323f1d6.

Reason for revert: fxb/64156

Original change's description:
> [a11y] Adds callback to notify screen reader when TTS engine connects.
>
> This change adds a mechanism for the screen reader to be notified when
> the TTS engine is ready to produce output.
>
> TEST: fx test a11y_lib_tests; fx test a11y_tests
> Change-Id: I03e501ce1e99ec88ee02cd6cd2ffad9c90612b15
> Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/445219
> Reviewed-by: Lucas Radaelli <lucasradaelli@google.com>
> Testability-Review: Lucas Radaelli <lucasradaelli@google.com>
> Commit-Queue: Alexander Brusher <abrusher@google.com>

TBR=abrusher@google.com,lucasradaelli@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: I94a0059e6e5159f7a0bb9e824bf6aaa062d7cd3d
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/450195
Reviewed-by: Alexander Brusher <abrusher@google.com>
Commit-Queue: Alexander Brusher <abrusher@google.com>