Remove obsolete bug.

The original bug is no longer relevant, and the FIXME here
contains lots of obsolete information.

PiperOrigin-RevId: 249924036

pkg/sentry/fs/dentry.go

@@ -83,9 +83,6 @@ type DirCtx struct {
 	attrs map[string]DentAttr
 
 	// DirCursor is the directory cursor.
-	// TODO(b/67778717): Once Handles are removed this can just live in the
-	// respective FileOperations implementations and not need to get
-	// plumbed everywhere.
 	DirCursor *string
 }