Remove dead RenderbufferStorage::getDepthBuffer. This code was unused. BUG=angle:732 Change-Id: I764d7c5624bdb81650e05a22c4ef0431c2002b18 Reviewed-on: https://chromium-review.googlesource.com/217331 Reviewed-by: Brandon Jones <bajones@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>