In the initial design for $rankFusion, we allowed users to optionally input an integer rankConstant as a weight term to RRF. Since then, we've decided to defer 'rankConstant' syntax to future consideration and will instead use a fixed weight multiplier of 60. We need to remove this field from the RankFusionInputsSpec idl definition. 

We will also need to update all relevant rankFusion tests.

UPDATE:
Design review is still happening. So, when completing this ticket, please consult the technical design document to see the finalized syntax and determine if any additional changes need to be made. At the time of writing this: 'rankConstant' needs to be removed and 'weights' needs to be added (which will require idl mods as well as updated tests). This ticket tracks only the work to remove 'rankConstant.' Adding 'weights' is tracked in SERVER-94603