LU-2920 autoconf: delete unused LB_LINUX_MODPOST

LB_LINUX_MODPOST does not seem to be used anywhere.

Signed-off-by: Minh Diep <minh.diep@intel.com>
Change-Id: Ibec2002ae8b74b9c6f1f6df82a35d44bd00916dc
Reviewed-on: http://review.whamcloud.com/5615
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Hudson
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>