Removes the last use of sw::Event in Vulkan.

Bug: b/140546382
Change-Id: I5de63c52e35756dc6d7e4c6cbb77669ea3925d79
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38208Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>