Drop the collection after removing the partially written chunks for mapReduce in mongos_manual_intervention_actions.js

As of SERVER-33538, we no longer allow mapReduce to be called with an existing unsharded output collection. If the shardCollection command fails (due to a stepdown, etc.) on the first attempt of mapReduce, the collection will be created but not sharded so we will fail the command entirely. We should drop the collection in addition to cleaning up the partially written chunks in the case of shardCollection failing during mapReduce.