MongoDB shouldn't do an initial projection for :default date bucketing

[camsaul]

Given a query like

(mt/dataset attempted-murders
  (mt/mbql-query attempts
    {:aggregation [[:count]]
     :filter      [:time-interval $datetime :last :month]}))

Which end up with a native query like this: (including #14835 optimizations):

[{"$project" {"datetime~~~default"
              {:$let {:vars {:column "$datetime"}, :in :$$column}}}}
 {"$match"
  {"$and"
   [{"datetime~~~default" {"$gte" {:___date "2021-01"}}}
    {"datetime~~~default" {"$lt" {:___date "2021-02"}}}]}}
 {"$group" {"_id" nil, "count" {"$sum" 1}}}
 {"$sort" {"_id" 1}}
 {"$project" {"_id" false, "count" true}}]

That initial projection is going to destroy potential usage of indexes, and we don't need it at all, since :default just means we explicitly do not want any extra temporal bucketing.

This is a subset of #4216. I fixed most of #4216 in #11832, but cases such as these are still causing initial projections... by solving this specific case we may be able to finally close #4216 entirely.

[camsaul]

Implemented by #14840