CLN: remove sample_time attributes from benchmarks
Very minor benchmark cleanup now that I was on it...

These attributes appear to have been brought in by direct replacement of a
deprecated ASV attribute goal_time -> sample_time, but the two have
different semantics (the value sample_time=0.2 is 20x the default).
Increasing this from the default value is probably not necessary and
increases runtime (here 3min vs 17min for rolling.*), so it's better to restore it to defaults.