LU-2800 autoconf: remove LIBCFS_SOCK_MAP_FD_2ARG

v2.6.27 added a second argument to sock_map_fd. Earlier versions are
out of scope so we can remove the test for the number of arguments.

Signed-off-by: James Simmons <uja.ornl@gmail.com>
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Change-Id: I4e12da8c102b5d682db9658187330ae601b00a21
Reviewed-on: http://review.whamcloud.com/5406
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>