Remove count_by from UsageData#count

This change was introduced in dd85269a but it hasn't been used.

lib/gitlab/usage_data.rb

@@ -228,8 +228,8 @@ module Gitlab
         {} # augmented in EE
       end
 
-      def count(relation, count_by: nil, fallback: -1)
-        count_by ? relation.count(count_by) : relation.count
+      def count(relation, fallback: -1)
+        relation.count
       rescue ActiveRecord::StatementInvalid
         fallback
       end

spec/lib/gitlab/usage_data_spec.rb

@@ -339,12 +339,6 @@ describe Gitlab::UsageData do
       expect(described_class.count(relation)).to eq(1)
     end
 
-    it 'returns the count for count_by when provided' do
-      allow(relation).to receive(:count).with(:creator_id).and_return(2)
-
-      expect(described_class.count(relation, count_by: :creator_id)).to eq(2)
-    end
-
     it 'returns the fallback value when counting fails' do
       allow(relation).to receive(:count).and_raise(ActiveRecord::StatementInvalid.new(''))