Simplify calling OS settings in quick_answers_ui_controller.cc

`OSSettingsType` enum and `ShowOSSettings()` function were introduced in
[1] in order to avoid having lacros-compiled code referring to //ash
constants (in this case ::kSystemPreferencesSectionPath and
::kSearchSubpagePath).

Now that the Lacros support has been removed, this CL cleans things up
in that regard.

[1] https://crrev.com/c/5846452

R=hidehiko@chromium.org, neis@chromium.org, yawano@google.com

Bug: b:365588558, 374253370, 373971535
Change-Id: Ia732413fe9eac31dc1884d1ca76778d9d9b70ee1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6088396
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Yuki Awano <yawano@google.com>
Cr-Commit-Position: refs/heads/main@{#1395493}