LU-12561 kernel: Remove RHEL6 kernel configs

LU-12561 kernel: Remove RHEL6 kernel configs

First in a series of patches to remove RHEL6 support, this
removes the kernel configs.

This should be a build only change, so trivial testing
should be OK.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I0b68754f8921b82e8d0c5eaa321d58187da49a70
Reviewed-on: https://review.whamcloud.com/35544
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>