[ChromeOS] Nuke SplitSettingsSync from c/b/c/preferences_unittest.cc

Bug: 1227693
Change-Id: I1bb94937d7ede3d389b90f024800aebf33f896e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3132985


Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#919126}