staging/lustre/autoconf: remove LIBCFS_HAVE_IS_COMPAT_TASK test

is_compat_task has been defined on all arches since v2.6.29.
We can remove the test and dead code.

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-2800
Lustre-change: http://review.whamcloud.com/5393
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>