Remove the reference to `WalkdirLoaderHandle` in vfs documentation
This structure was deleted in commit #dad1333
//! free-form via list of globs, it's up to the `Handle` to interpret the globs
//! in any specific way.
//!
-//! A simple `WalkdirLoaderHandle` is provided, which doesn't implement watching
-//! and just scans the directory using walkdir.
-//!
//! VFS stores a flat list of files. `FileSet` can partition this list of files
//! into disjoint sets of files. Traversal-like operations (including getting
//! the neighbor file by the relative path) are handled by the `FileSet`.
