Remove fields from count() queries when associations are contained.

Strip out any fields defined in contained associations as fetching
fields and throwing away the results is wasteful and can lead to slow
downs in larger datasets.

Refs #5955

src/ORM/Query.php

@@ -507,8 +507,16 @@ public function count()
         $complex = $complex || count($query->clause('union'));
 
         if (!$complex) {
+            $cleanContain = [];
+            foreach ($query->contain() as $alias => $contain) {
+                unset($contain['fields']);
+                $cleanContain[$alias] = $contain;
+            }
+            $query->contain(null, true);
+
             $statement = $query
                 ->select($count, true)
+                ->contain($cleanContain)
                 ->autoFields(false)
                 ->execute();
         } else {