LU-13164 uapi: remove unused LUSTRE_DIRECTIO_FL

LU-13164 uapi: remove unused LUSTRE_DIRECTIO_FL

The LUSTRE_DIRECTIO_FL was added based on the upstream FS_DIRECTIO_FL
flag in the hopes that it might be useful, but it has since been
removed from the upstream in kernel commit v4.4-rc4-22-g68ce7bfcd995
and replaced by FS_VERITY_FL using the same value in kernel commit
v5.3-rc2-4-gfe9918d3b228, which we are much more likely to use.

Since LUSTRE_DIRECTIO_FL was unused, there is no risk to remove it.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I49e915612636a674a86d25be5d91a042693ebbe5
Reviewed-on: https://review.whamcloud.com/37295
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Arshad Hussain <arshad.super@gmail.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>