[DML EP] Revert DML's cpu fallback logic (#13605)
### Description
Revert DML's CPU fallback logic from
https://github.com/microsoft/onnxruntime/pull/13442.
### Motivation and Context
Although the logic works great in many models that have good DML
coverage, it makes perf worse in some models where many operators are
missing DML coverage (e.g. int64). Overall, the right fix seems to
instead implement the operator on DML even though it almost always falls
back to the CPU, just for the sake of having a registration.
