Fix invalid escape sequence (#17145)
### Description
- Removed one unused import
- Escaped a backslash in a path
### Motivation and Context
I see this `DeprecationWarning` when I import `onnxruntime`:
```
onnxruntime/capi/_pybind_state.py:28: DeprecationWarning: invalid escape sequence '\S'
"(other than %SystemRoot%\System32), "
```
A future version of Python (maybe 3.13?) will raise a `SyntaxError` for
invalid escape sequences.
