drivers: lustre: obdclass: simplify unregister_shrinker() usage

lu_global_fini() explicitly uses knowledge about shrinker's
internals to make decision about calling of unregister_shrinker().
Now this check was integrated into unregister_shrinker(),
so it is safe to call it against unregistered shrinker.

Signed-off-by: Aliaksei Karaliou <akaraliou.dev@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>