LU-6142 llite: remove lli_readdir_mutex

LU-6142 llite: remove lli_readdir_mutex

This mutex is initialized but never used, so
remove it.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.com>
Change-Id: I8df34923c44892dbf3a4e0bb603209cfe28adb1b
Reviewed-on: https://review.whamcloud.com/35884
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Arshad Hussain <arshad.super@gmail.com>