Remove sw::TaskEvents and sw::WaitGroup, replace this with sw::CountedEvent.

See b/173784261 for details.

Fixes: b/173784261
Change-Id: I21fb69c810558a1929bba5cc46f106d9d4e51c4b
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50628
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Presubmit-Ready: Ben Clayton <bclayton@google.com>