Clean up DeviceO2TSDelegate

This CL is a part of moving access token management to
OAuth2AccessTokenManager.

It is specifically the last step folding
DeviceO2TSDelegate into DeviceO2TS now that the
latter is no longer an O2TS subclass.

It moves GetURLLoaderFactory() and CreateAccessTokenFetcher()
to DeviceO2TS and eliminating DeviceO2TS inheriting from
DeviceOAuth2TokenServiceDelegate::ValidationStatusDelegate
since DeviceO2TS can handle ReportServiceError() by itself.

With this CL, device_oauth2_token_service_delegate.cc/h
are removed from the source tree.

Bug: 967598
Change-Id: I5e391cee94a67185c6e83aab8fccb54cf4b215a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1697002
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676378}