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.

spec/lib/geozone_stats_spec.rb

@@ -1,44 +0,0 @@
-require "rails_helper"
-
-describe GeozoneStats do
-  let(:winterfell) { create(:geozone, name: "Winterfell") }
-  let(:riverlands) { create(:geozone, name: "Riverlands") }
-
-  describe "#name" do
-    let(:stats) { GeozoneStats.new(winterfell, []) }
-
-    it "returns the geozone name" do
-      expect(stats.name).to eq "Winterfell"
-    end
-  end
-
-  describe "#count" do
-    before do
-      2.times { create(:user, geozone: winterfell) }
-      1.times { create(:user, geozone: riverlands) }
-    end
-
-    let(:winterfell_stats) { GeozoneStats.new(winterfell, User.all) }
-    let(:riverlands_stats) { GeozoneStats.new(riverlands, User.all) }
-
-    it "counts participants from the geozone" do
-      expect(winterfell_stats.count).to eq 2
-      expect(riverlands_stats.count).to eq 1
-    end
-  end
-
-  describe "#percentage" do
-    before do
-      2.times { create(:user, geozone: winterfell) }
-      1.times { create(:user, geozone: riverlands) }
-    end
-
-    let(:winterfell_stats) { GeozoneStats.new(winterfell, User.all) }
-    let(:riverlands_stats) { GeozoneStats.new(riverlands, User.all) }
-
-    it "calculates percentage relative to the amount of participants" do
-      expect(winterfell_stats.percentage).to eq 66.667
-      expect(riverlands_stats.percentage).to eq 33.333
-    end
-  end
-end