[Accessibility] Clean up headers in RenderAccessibilityImpl

This CL removes no-longer-used headers from
RenderAccessibilityImpl and related files. A few consumers of
render_accessibility_impl.h needed to have some additional
includes added for IWYU.

This CL also removes load_inline_text_boxes_ids_ from
RenderAccessibilityImpl, as that isn't used anywhere.

No behavioral changes are expected.

AX-Relnotes: n/a
Change-Id: Ifbc871477830301d89654c393d2e7e77697ad2ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5901000
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Kurt Catti-Schmidt <kschmi@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1363135}