Vulkan: Remove serial from RenderPassHelper We don't delete RenderPass's so no need to keep a serial. This simplifies things for coming threading code. Bug: b/169788986 Change-Id: I2577b17bc1f6af163725389589d7cd62d09eea13 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2468538 Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>