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

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

spec/lib/email/receiver_spec.rb

@@ -1550,10 +1550,12 @@ RSpec.describe Email::Receiver do
       handler_calls = 0
       handler =
         proc do |topic|
-          expect(topic.incoming_email_addresses).to contain_exactly(
-            "discourse@bar.com",
-            "category@foo.com",
-          )
+          expect(
+            [
+              topic.incoming_email.first.from_address,
+              topic.incoming_email.first.to_addresses_split,
+            ].flatten,
+          ).to contain_exactly("discourse@bar.com", "category@foo.com")
           handler_calls += 1
         end