Various cleanups: ppltasks.cpp

[StephanTLavavej]

I've validated this with an internal build and test run. I know there's not a lot of value in cleaning up old src files, but this eliminates some complexity and I believe it's safe.

• RtlCaptureStackBackTrace is universally available.
  • It might not have been when this was originally implemented, but it's now documented as "Target Platform: Universal", and this aligns with its actual declaration.
• Move GUID into Concurrency::details within #if defined(_CRT_APP) || defined(UNDOCKED_WINDOWS_UCRT).
  • It's static, so there's no ABI impact.
• We don't need <mutex> - only <functional> for std::function and <stdexcept> for std::runtime_error.
• Drop "ASYNC TODO" comment.
  • We're never going to change this legacy behavior.

[StephanTLavavej]

I'm speculatively mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.