In a 6WR meeting with the product team, we agreed that the 'sum' combination.method option is redundant with the 'avg' combination method.

This is because the 'avg' of the combined scores is the same as the 'sum' divided by a constant (the number of input pipelines), and thus the relative order of the output documents in $scoreFusion will never be different between these two combination methods, and are redundant.

Remove the 'sum' combination method and all associated logic and tests.