refactor: remove coerce_to_dataframe from execute_udaf_node_groupby in pandas backend

[icexelloss]

In #2487, we added a this to execute_udaf_node_groupby to coerce the output to the right shapes when executing struct UDF, however, this is not great because this does similar things to pandas.execution.util.coerce_to_output but we cannot use pandas.execution.util.coerce_to_output because we are in the inner loop. (pandas.execution.util.coerce_to_output only works after getting the result from inner loop).

Would be nice to clean this up and conform to a single coercion function.

[cpcloud]

PRs welcome! We're trying to keep the issue backlog tidy, so going to close this out. Feel free to submit refactors as PRs directly.