DAL: use small value for number_of_shards in slow AI tests

What

We use a smaller value of the DAL parameter number_of_shards in two particular AI slow tests.

We simplify the DAL committee type by removing the unused mapping.

Why and How

Computing the DAL committee takes a bit of time, around 1ms for the currently default value number_of_shards = 2048, and this adds up when baking for a long time, around 14min for the given tests. To make the tests run faster we use a smaller value of the parameter.

Computing the unused mapping also takes some time. So we remove it to further reduce the computation time.