Remove some unused code from `csrc/runtime` (#5785)
* delete nccl_distributed
* remove async_task
* remove unique
* Remove hashing
* more random cleanup
* formatting
* remove util.cc
* Revert "remove unique"
This reverts commit ebe4567b07b5fca72c6952b8ae30d4f404d3c8e2.
* Use upstream Unique
