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/+/2468538Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>