Angle: Remove obsolete requires_sdk_api_level_23 from BUILD.gn.
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 <36f5580f63d0a3eacc17e89a61d11e2f646630d2@google.com>
Commit-Queue: Yuly Novikov <36f5580f63d0a3eacc17e89a61d11e2f646630d2@google.com>

ppy/angle,ppy/angle,ppy/angle,ppy/angle