[clang] [libc] [openmp] [Clang] Remove the old LLVM offloading driver entirely (PR #211924)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> This PR completely removes the support so we stop needing to support both when making updates to the driver.

Thank you for paying down this tech debt! I'm in favor of this, once we've addressed:

> We're still running with the old offloading driver, so the patch will be a bit of a showstopper for us.

https://github.com/llvm/llvm-project/pull/211924