[SPARC]: Kill CHILD_MAX.

It's definition is wrong (-1 means "no limit" not 999),
only the Sparc SunOS/Solaris compat code uses it, so
let's just kill it off completely from limits.h and
all referencing code.

Noticed by Ulrich Drepper.

Signed-off-by: David S. Miller <davem@davemloft.net>