Remove the split extension traits

[MabezDev]

Initially suggested by @brainstorm, thanks for feedback!

I'm pretty sure we don't need them anymore, and they create confusion in documentation because it's a trait method.

• GPIO Extension - only used internally anyways
  • Eliminate the GpioExt trait #1496
• SYSTEM Extension - no longer needed as its always called SYSTEM, we can just have SystemParts (bike shed the name) take SYSTEM in the constructor.
  • Create virtual peripherals for CPU control and radio clocks #1428
  • Remove the SystemExt trait and rename SystemParts to SystemControl #1495

[jessebraham]

Thanks, meant to open an issue for this but forgot. I actually have a local branch from some time back where I had refactored away the SYSTEM.split() bits, I will try to revive it.

[jessebraham]

With #1428 now merged, once #1495 has been merged then the work on SYSTEM will be complete. Will look into the GPIO extension trait next, which should close this issue.