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).

include/onnxruntime/core/graph/graph.h

@@ -17,12 +17,12 @@
 #include "core/common/gsl.h"
 
 #include "core/common/common.h"
+#include "core/common/path_string.h"
 #include "core/common/const_pointer_container.h"
 #if !defined(ORT_MINIMAL_BUILD)
 #include "core/common/inlined_containers.h"
 #endif
 #include "core/common/inlined_containers_fwd.h"
-#include "core/common/path.h"
 #include "core/common/span_utils.h"
 #include "core/common/status.h"
 #include "core/common/logging/logging.h"