[Read aloud] Just remove the formatting when speech ends

Previously we were updating the entire content completely but this can
be disorienting since it scrolls you back to the top.

Bug: 40927698
Change-Id: I7c1f6cb34d87e1c1a30417e300bb14e285d20d3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6473454
Commit-Queue: Kristi Saney <kristislee@google.com>
Reviewed-by: Lauren Winston <lwinston@google.com>
Commit-Queue: Lauren Winston <lwinston@google.com>
Auto-Submit: Kristi Saney <kristislee@google.com>
Cr-Commit-Position: refs/heads/main@{#1449058}