Rules using Aggregate might check for simple grouping sets incorrectly

CALCITE-1069 removed the indicator columns for Aggregate operators. In some places, the indicator boolean check was replaced by the following check: aggregate.getGroupSets().size() > 1. However, that check is incomplete, it should have been replaced by aggregate.getGroupType() != Group.SIMPLE.

For instance : https://github.com/apache/calcite/blob/master/core/src/main/java/org/apache/calcite/rel/rules/AggregateProjectMergeRule.java#L91