Remove unused parameter in lookup_var_attr_stats

Remove unused parameter in lookup_var_attr_stats

The parameter 'rel' in lookup_var_attr_stats was once used to draw an
ERROR when ANALYZE failed to acquire sufficient data to build extended
statistics.  bf2a691e0 changed the logic to raise a WARNING in the
caller instead.  As a result, this parameter is no longer needed and
can be removed.  Since this is a static function, we can always easily
reintroduce the parameter if it's ever needed in the future.

Author: Ilia Evdokimov
Reviewed-by: Fabrízio de Royes Mello
Discussion: https://postgr.es/m/b3880f22-5808-4206-88d4-1553a81c3440@tantorlabs.com