WebGPU: Remove last references to cpp API Bug: angleproject:414831373 Change-Id: I25c03de40c579bf984d9a905ecf655c82d239811 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6512149 Reviewed-by: Matthew Denton <mpdenton@chromium.org> Reviewed-by: Liza Burakova <liza@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>