[Start] Clean up START_SURFACE_ON_TABLET related code

Since this flag has been enabled by default, we cleaned up any code
related to this flag.

Bug: 324879903
Change-Id: Ic6856acbf9763fe0874040fa3c26b908dbb6c0b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5378391


Commit-Queue: Xinyi Ji <xinyiji@chromium.org>
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279970}