[ROCM] remove more rocm ep code (#26712)
### Description
Remove USE_ROCM and rocm ep related code.
Public APIs (C/C++ APIs and corresponding C# native methods) are not
touched.
### Motivation and Context
Follow up on https://github.com/microsoft/onnxruntime/pull/25181 and
https://github.com/microsoft/onnxruntime/pull/26697
