staging: lustre: get rid of ldebugfs_remove()

It was just a dumb wrapper around debugfs_remove_recursive() so just
call the function properly.  Also, there is no need to set the dentry to
NULL, it's gone, who cares about it anymore...

Cc: Oleg Drokin <oleg.drokin@intel.com>
Cc: Andreas Dilger <andreas.dilger@intel.com>
Cc: James Simmons <jsimmons@infradead.org>
Cc: NeilBrown <neilb@suse.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Roman Storozhenko <romeusmeister@gmail.com>
Cc: Aastha Gupta <aastha.gupta4104@gmail.com>
Cc: Ben Evans <bevans@cray.com>
Cc: Quentin Bouget <quentin.bouget@cea.fr>
Cc: Arvind Yadav <arvind.yadav.cs@gmail.com>
Cc: Arushi Singhal <arushisinghal19971997@gmail.com>
Cc: Patrick Farrell <paf@cray.com>
Cc: Aliaksei Karaliou <akaraliou.dev@gmail.com>
Cc: Mathias Rav <mathiasrav@gmail.com>
Cc: Andriy Skulysh <andriy.skulysh@seagate.com>
Cc: Dafna Hirschfeld <dafna3@gmail.com>
Cc: "Gustavo A. R. Silva" <garsilva@embeddedor.com>
Cc: Bob Glosman <bob.glossman@intel.com>
Cc: lustre-devel@lists.lustre.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>