LU-17606 build: remove el7.[678] kernel patch series

LU-17606 build: remove el7.[678] kernel patch series

Remove the kernel patches for el7.[678] along with another
obsolete configure check in the ldiskfs tree.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ie5ea5dd96fb7a9e7315e9a80117dad5de63ebbe5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54324
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>