Delete path.h (#21211)
### Description
Delete path.h and replace all occurrences of onnxruntime::Path with
std::filesystem::path.
Previously we couldn't use C++17's std::filesystem because it was not
supported in iOS 12(which was released in 2018). Now we dropped the
support for iOS 12.
### Motivation and Context
To simplify code. For example, if an EP wants to use the Path class, now
it can directly use it without going through a wrapper. And the standard
implementation can handle various path types better. (We didn't take
much consideration on UNC path, "/" as a path separator on Windows,
etc).
