LU-3080 misc: remove checks for pre-3.10 kernels

LU-3080 misc: remove checks for pre-3.10 kernels

The oldest supported client kernel is el7.9 3.10.x, and even that
is old, so no need to keep checks for 2.6.x kernel versions around.

Test-Parameters: trivial testlist=replay-dual
Test-Parameters: testlist=replay-single
Test-Parameters: testlist=sanity-flr env=ONLY=50d
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I0db781c048334e6ef6df102b100d29e13c64fd25
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50814
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Alex Deiter <alex.deiter@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>