Reactor: Replace ArgI for std::tuple_element

Reactor: Replace ArgI for std::tuple_element

C++ now provides this template magic so you don't have to.

Also remove Arg(Function<Return(Arguments...)> &) function that wasn't referenced and wouldn't compile if it were.

Bug: b/126126820
Bug: b/129403963
Change-Id: I939e4f7cb95c32b8e663df7da6211460c5de605f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/25749
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <headlessclayton@gmail.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>