Remove WindowSurfaceVk::swapImpl prototype. This method was removed in a prior patch. BUG=angleproject:1898 Change-Id: I2ab3c36bbef4ce6c978cd4c09cc191c4e7ff7991 Reviewed-on: https://chromium-review.googlesource.com/677547 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>