Remove _fdopen from PAL (#77272)

This function is never used.

src/coreclr/pal/inc/pal.h

@@ -4408,7 +4408,6 @@ PALIMPORT int __cdecl PAL_ferror(PAL_FILE *);
 PALIMPORT PAL_FILE * __cdecl PAL_fopen(const char *, const char *);
 PALIMPORT int __cdecl PAL_setvbuf(PAL_FILE *stream, char *, int, size_t);
 
-PALIMPORT PAL_FILE * __cdecl _fdopen(int, const char *);
 PALIMPORT PAL_FILE * __cdecl _wfopen(const WCHAR *, const WCHAR *);
 
 /* Maximum value that can be returned by the rand function. */

src/coreclr/pal/src/cruntime/file.cpp

@@ -123,7 +123,7 @@ static LPSTR MapFileOpenModes(LPSTR str , BOOL * bTextMode)
     The PAL does not support this behavior. */
     if (NULL != strchr(str,'D'))
     {
-        ASSERT("The PAL doesn't support the 'D' flag for _fdopen and fopen.\n");
+        ASSERT("The PAL doesn't support the 'D' flag for fopen.\n");
         return NULL;
     }
 
@@ -193,63 +193,6 @@ static BOOL WriteOnlyMode(FILE* pFile)
 }
 #endif //UNGETC_NOT_RETURN_EOF
 
-/*++
-Function:
-  _fdopen
-
-see MSDN
-
---*/
-PAL_FILE *
-__cdecl
-_fdopen(
-    int handle,
-    const char *mode)
-{
-    PAL_FILE *f = NULL;
-    LPSTR supported = NULL;
-    BOOL bTextMode = TRUE;
-
-    PERF_ENTRY(_fdopen);
-    ENTRY("_fdopen (handle=%d mode=%p (%s))\n", handle, mode, mode);
-
-    _ASSERTE(mode != NULL);
-
-    f = (PAL_FILE*)PAL_malloc( sizeof( PAL_FILE ) );
-    if ( f )
-    {
-        supported = MapFileOpenModes( (char*)mode , &bTextMode);
-        if ( !supported )
-        {
-            PAL_free(f);
-            f = NULL;
-            goto EXIT;
-        }
-
-        f->bsdFilePtr = (FILE *)fdopen( handle, supported );
-        f->PALferrorCode = PAL_FILE_NOERROR;
-        /* Make sure fdopen did not fail. */
-        if ( !f->bsdFilePtr )
-        {
-            PAL_free( f );
-            f = NULL;
-        }
-
-        PAL_free( supported );
-        supported = NULL;
-    }
-    else
-    {
-        ERROR( "Unable to allocate memory for the PAL_FILE wrapper!\n" );
-    }
-
-EXIT:
-    LOGEXIT( "_fdopen returns FILE* %p\n", f );
-    PERF_EXIT(_fdopen);
-    return f;
-}
-
-
 /*++
 
 Function :

src/coreclr/pal/tests/palsuite/README.txt

@@ -13,7 +13,6 @@
 3.2  Other Notes
 
 4.   ADDITIONAL NOTES ON TESTING/SPECIFIC TEST CASE ISSUES
-     C_runtime: _fdopen testing issues
      File_IO: getfilesize/test1, setfilepointer/test(5,6,7)
      File_IO: gettempfilename(a,w)/test2
      File_IO: setfileattributesa/test(1,4), setfileattributesw/test(1,4)
@@ -53,33 +52,6 @@ See notes in section 4 on the following test cases if running automated tests:
 4. ADDITIONAL NOTES ON TESTING/SPECIFIC TEST CASE ISSUES
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-C Runtime: _fdopen testing issues
-
-There is a very specific manner in which _fdopen has been documented to work,
-this will determine how the function will be tested.
-
-_fdopen takes a parameter of a c run-time file handle, to open a stream to the
-file. This file handle can only be returned from the _open_osfhandle function.
-The Rotor documentation states that _open_osfhandle will only return a
-READ-ONLY file handle, from an operating-system file handle returned from
-CreatePipe().
-
-With these restrictions _fdopen will only be tested with a mode of read(r).
-The other modes are not possible to test. _open_osfhandle returns an error
-when attempting to open a write pipe handle in a mode of read-only. As well,
-it is not possible to read and write to the same handle that is returned from
-CreatePipe().
-
-The modes that will not be tested are as follows:
-
-    "w" -  Opens an empty file for writing.
-    "a" -  Opens for writing at the end of the file (appending).
-    "r+" - Opens for both reading and writing.
-    "w+" - Opens an empty file for both reading and writing.
-    "a+" - Opens for reading and appending.
-
-
-
 File_IO: getfilesize/test1, getfilesizeex/test1 setfilepointer/test(5,6,7)
 
 These tests cases create a large number of temporary files which require