Delete path.h

[snnn]

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).