chore: leave a note to remove callback-form of aggregate in 4.x

Author: mbroadst

lib/collection.js

@@ -1758,6 +1758,7 @@ Collection.prototype.aggregate = function(pipeline, options, callback) {
     options
   );
 
+  // TODO: remove this when NODE-2074 is resolved
   if (typeof callback === 'function') {
     callback(null, cursor);
     return;

lib/db.js

@@ -336,6 +336,7 @@ Db.prototype.aggregate = function(pipeline, options, callback) {
     options
   );
 
+  // TODO: remove this when NODE-2074 is resolved
   if (typeof callback === 'function') {
     callback(null, cursor);
     return;