Angle: Remove obsolete requires_sdk_api_level_23 from BUILD.gn. Will be removed upstream. BUG=chromium:688465 Change-Id: If50ac3624a5f692755797eaabcc0c7d6e7408655 Reviewed-on: https://chromium-review.googlesource.com/1244079 Reviewed-by: Yuly Novikov <ynovikov@google.com> Commit-Queue: Yuly Novikov <ynovikov@google.com>