staging/lustre/llite: get rid of unused ll_super_blocks list

ll_super_blocks became unused quite a while ago with switch
to the new CLIO code.
So this patch removes the list, ll_sb_lock spinlock that guards it
and superblock info ll_list linkage.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>