[Extensions] Remove the preferencesPrivate API

The preferencesPrivate API is no longer used. Remove it.

Bug: 593166
Change-Id: I5d56c3c79d0423b6534a7298f3d5103a09bc6707
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529119
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642383}
20 files changed