fs/dcache: Remove stale comment from dentry_kill()

Commit 0d98439e ("vfs: use lockred "dead" flag to mark unrecoverably
dead dentries") removed the `ref' parameter in dentry_kill() but its
documentation remained. Remove it.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -591,7 +591,6 @@ static void __dentry_kill(struct dentry *dentry)
 /*
  * Finish off a dentry we've decided to kill.
  * dentry->d_lock must be held, returns with it unlocked.
- * If ref is non-zero, then decrement the refcount too.
  * Returns dentry requiring refcount drop, or NULL if we're done.
  */
 static struct dentry *dentry_kill(struct dentry *dentry)