LU-12477 kernel: remove dev_read_only handling

LU-12477 kernel: remove dev_read_only handling

To test disk failures Lustre developed a method for ldiskfs to
make the backend block device appears to have failed. Modern
kernels offer the same thing with dm-flakey. Lustre has moved
to dm-flakey for some time so the old dev_read_only can now be
removed.

Change-Id: I3d5887ab8c0daf1c94c9069bcdd58786b2819ac0
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/38070
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>