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/lib/geozone_stats.rb

@@ -1,24 +0,0 @@
-class GeozoneStats
-  attr_reader :geozone, :participants
-
-  def initialize(geozone, participants)
-    @geozone = geozone
-    @participants = participants
-  end
-
-  def geozone_participants
-    participants.where(geozone: geozone)
-  end
-
-  def name
-    geozone.name
-  end
-
-  def count
-    geozone_participants.count
-  end
-
-  def percentage
-    PercentageCalculator.calculate(count, participants.count)
-  end
-end

app/lib/percentage_calculator.rb

@@ -1,7 +0,0 @@
-module PercentageCalculator
-  def self.calculate(fraction, total)
-    return 0.0 if total.zero?
-
-    (fraction * 100.0 / total).round(3)
-  end
-end