Since SERVER-30871, we permit blocking stages to run on mongoS if allowDiskUse is false. The $facet stage was excluded from the list of eligible blocking stages because it requires the injection of a MongodInterface, even when none of its pipelines actually use it; this caused complications in the event that the $facet was run on mongoS.

Following SERVER-29609, however, we now have an equivalent MongosProcessInterface for cases where a pipeline is executing on mongoS, which obviates the issues mentioned above. We should allow $facet to run on mongoS again.