vfs: remove unused exports from fd.js and stats.js

- Remove unused isVirtualFd() function from fd.js
- Remove unused getOpenFdCount() function from fd.js
- Remove unused kDefaultBlockSize export from stats.js (still used
  internally)

These functions were exported but never used anywhere in the codebase.

PR-URL: #61478

Author: mcollina

lib/internal/vfs/fd.js

@@ -133,29 +133,10 @@ function closeVirtualFd(fd) {
   return openFDs.delete(fd);
 }
 
-/**
- * Checks if a file descriptor is a virtual fd.
- * @param {number} fd The file descriptor number
- * @returns {boolean}
- */
-function isVirtualFd(fd) {
-  return openFDs.has(fd);
-}
-
-/**
- * Gets the count of open virtual file descriptors.
- * @returns {number}
- */
-function getOpenFdCount() {
-  return openFDs.size;
-}
-
 module.exports = {
   VirtualFD,
   VFS_FD_BASE,
   openVirtualFd,
   getVirtualFd,
   closeVirtualFd,
-  isVirtualFd,
-  getOpenFdCount,
 };

lib/internal/vfs/stats.js

@@ -192,5 +192,4 @@ module.exports = {
   createFileStats,
   createDirectoryStats,
   createSymlinkStats,
-  kDefaultBlockSize,
 };