Extensions: Remove screenlockPrivate API.

This is no longer used.

R=tbarzic@chromium.org

Bug: 904892
Test: none
Change-Id: Iac3088bdc2b198d0d5b37ab9cf18fba5c4c704c8
Reviewed-on: https://chromium-review.googlesource.com/c/1334331
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: James Hawkins <jhawkins@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608821}
20 files changed