myrialang: switch back to SingleGroupByAggregate
myrialang: switch back to SingleGroupByAggregate

We fixed the NoBuffer/regular on the Myria java side as of

uwescience/myria@47f69ef

Signed-off-by: Daniel Halperin <1a02732a15f637c31107256cbee5819e36aa7c67@cs.washington.edu>

uwescience/raco,uwescience/raco,uwescience/raco,uwescience/raco,uwescience/raco