LU-12477 kernel: remove < 2.6.39 kernel support

LU-12477 kernel: remove < 2.6.39 kernel support

We no longer support kernels 2.6.38 or older so we can remove all
the various autoconf test and simplify the lustre code. Only
leave LC_EXPORT_TRUNCATE_COMPLETE_PAGE since 4.X kernel test are
linked to this test.

Change-Id: I0532507dd3310b7ef4f4e4201dfa8b47c2991163
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/35551
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>