DEV: Delete dead Topic#incoming_email_addresses code (#19970)

This code has been dead since b463a80cbfffa4aaa38e14cc674c4cd295468fd9,
we can delete it now.

app/models/topic.rb

@@ -1892,46 +1892,6 @@ class Topic < ActiveRecord::Base
       .first
   end
 
-  def incoming_email_addresses(group: nil, received_before: Time.zone.now)
-    email_addresses = Set.new
-
-    # TODO(martin) Look at improving this N1, it will just get slower the
-    # more replies/incoming emails there are for the topic.
-    self
-      .incoming_email
-      .where("created_at <= ?", received_before)
-      .each do |incoming_email|
-        to_addresses = incoming_email.to_addresses_split
-        cc_addresses = incoming_email.cc_addresses_split
-        combined_addresses = [to_addresses, cc_addresses].flatten
-
-        # We only care about the emails addressed to the group or CC'd to the
-        # group if the group is present. If combined addresses is empty we do
-        # not need to do this check, and instead can proceed on to adding the
-        # from address.
-        #
-        # Will not include test1@gmail.com if the only IncomingEmail
-        # is:
-        #
-        # from: test1@gmail.com
-        # to: test+support@discoursemail.com
-        #
-        # Because we don't care about the from addresses and also the to address
-        # is not the email_username, which will be something like test1@gmail.com.
-        if group.present? && combined_addresses.any?
-          next if combined_addresses.none? { |address| address =~ group.email_username_regex }
-        end
-
-        email_addresses.add(incoming_email.from_address)
-        email_addresses.merge(combined_addresses)
-      end
-
-    email_addresses.subtract([nil, ""])
-    email_addresses.delete(group.email_username) if group.present?
-
-    email_addresses.to_a
-  end
-
   def create_invite_notification!(target_user, notification_type, invited_by, post_number: 1)
     if UserCommScreener.new(
          acting_user: invited_by,