LU-15317 libcfs: Remove D_TTY

The D_TTY flag is almost entirely unused and certainly not
needed.  Remove it so we have a spare flag to use for
iotrace.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I1127cbcf6ee51adc07d560a8827fa1e32d16c90c
Reviewed-on: https://review.whamcloud.com/45751
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>