[llvm] [openmp] [Offload] Remove handling for device memory pool (PR #163629)

[Shilei Tian via llvm-commits]

https://github.com/shiltian commented:

Well, I'm not sure what our policy regarding using offload with libc. Is that required now? If not, it is okay for me to have our own implementation. Alternatively, even if we want to remove it, we want to document it somewhere that, if anyone wants to use device side dynamic allocation, they need to enable `libc` project; otherwise this will not work.

On the other hand, I think even our device runtime requires the support for dynamic allocation for the smart stack when static shared memory allocation runs out. This PR will make it completely not work.

https://github.com/llvm/llvm-project/pull/163629