Remove no longer needed calls to require_dependency

Not sure about when we stopped needing them, but all usages of
require_dependency definitely become obsolete after we started using
Zeitwerk in commit 5f24ee912.

We're also going to rename `Poll::Question::Answer` to
`Poll::Question::Option`, so the conflict of having two `Answer`
classes, that made us add this code in the first place, will not even
exist.

app/models/poll.rb

@@ -1,6 +1,4 @@
 class Poll < ApplicationRecord
-  require_dependency "poll/answer"
-
   include Imageable
   acts_as_paranoid column: :hidden_at
   include ActsAsParanoidAliases

db/dev_seeds/polls.rb

@@ -1,6 +1,3 @@
-require_dependency "poll/answer"
-require_dependency "poll/question/answer"
-
 section "Creating polls" do
   def create_poll!(attributes)
     poll = Poll.create!(attributes.merge(starts_at: 1.day.from_now, ends_at: 2.days.from_now))