Make Program::usesPointSize D3D-only. This test is only used in the D3D code, so we can remove this Impl method. If we need it in the future we can always support it more generally from the translator, or handle it differently on each Renderer. BUG=angleproject:1123 Change-Id: Ic4a22ad6cec93726195e68200fc25cc2956be44c Reviewed-on: https://chromium-review.googlesource.com/293822 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>