We moved this into the SwiftShader repo proper in:
https://swiftshader-review.googlesource.com/c/SwiftShader/+/38491

We can remove ANGLE's custom rules. Also removes the layer generation
script which was now unused. Also cleans up the Vulkan back-end
BUILD.gn a bit.

Bug: dawn:283
Change-Id: I490e8e8fc6fcadec58aca53a157ce7c4a47def3c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1932340Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>