Revert "Extract class to manage GeozoneStats"

Back in commit 383909e16, we said:

> Even if this class looks very simple now, we're trying a few things
> related to these stats. Having a class for it makes future changes
> easier and, if there weren't any future changes, at least it makes
> current experiments easier.

Since there haven't been any changes in the last 5 years and we've found
cases where using the GeozoneStats class results in a slightly worse
performance, we're removing this class. The code is now a bit easier to
read, and is consistent with the way we calculate participants by age.

This reverts commit 383909e16.

app/models/concerns/statisticable.rb

@@ -114,19 +114,23 @@ module Statisticable
   end
 
   def participants_by_geozone
-    geozone_stats.to_h do |stats|
+    geozones.to_h do |geozone|
+      count = participants.where(geozone: geozone).count
+
       [
-        stats.name,
+        geozone.name,
         {
-          count: stats.count,
-          percentage: stats.percentage
+          count: count,
+          percentage: calculate_percentage(count, total_participants)
         }
       ]
     end
   end
 
   def calculate_percentage(fraction, total)
-    PercentageCalculator.calculate(fraction, total)
+    return 0.0 if total.zero?
+
+    (fraction * 100.0 / total).round(3)
   end
 
   def version
@@ -178,10 +182,6 @@ module Statisticable
       Geozone.order("name")
     end
 
-    def geozone_stats
-      geozones.map { |geozone| GeozoneStats.new(geozone, participants) }
-    end
-
     def range_description(start, finish)
       if finish > 200
         I18n.t("stats.age_more_than", start: start)