[s13n] Remove unused header from child_account_service.cc

This CL removes unused header, signin_pref_names.h, from
child_account_service.cc.

Bug: 890795
Change-Id: If243590db397fd6d70cb5b23e38e500346d9f508
Reviewed-on: https://chromium-review.googlesource.com/c/1442074
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#626986}

chrome/browser/supervised_user/child_accounts/child_account_service.cc

@@ -29,7 +29,6 @@
 #include "components/pref_registry/pref_registry_syncable.h"
 #include "components/prefs/pref_service.h"
 #include "components/signin/core/browser/gaia_cookie_manager_service.h"
-#include "components/signin/core/browser/signin_pref_names.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/storage_partition.h"
 #include "services/identity/public/cpp/identity_manager.h"