Remove dead serializer

This became unused in
b9d72ce4cf (diff-5df9d1d16b9e9ac1724af5c6180ca86f7caeb6a4880d2f77ac25fbe757b597b9).

app/serializers/api/orders_by_distributor_serializer.rb

@@ -1,18 +0,0 @@
-module Api
-  class OrdersByDistributorSerializer < ActiveModel::Serializer
-    attributes :name, :id, :hash, :balance, :logo, :distributed_orders
-    has_many :distributed_orders, serializer: Api::OrderSerializer
-
-    def balance
-      object.distributed_orders.map(&:outstanding_balance).reduce(:+)
-    end
-
-    def hash
-      object.to_param
-    end
-
-    def logo
-      object.logo(:small) if object.logo?
-    end
-  end
-end