Remove unused answer_id field in poll_voters table

This field isn't used since commit 51be80eed, right after being
added in commit 5806d86e3.

db/migrate/20240511141119_remove_answer_id_from_poll_voters.rb

@@ -0,0 +1,5 @@
+class RemoveAnswerIdFromPollVoters < ActiveRecord::Migration[7.0]
+  def change
+    remove_column :poll_voters, :answer_id, :integer
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.0].define(version: 2024_04_24_013913) do
+ActiveRecord::Schema[7.0].define(version: 2024_05_11_141119) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
   enable_extension "plpgsql"
@@ -1208,7 +1208,6 @@ ActiveRecord::Schema[7.0].define(version: 2024_04_24_013913) do
     t.integer "age"
     t.string "gender"
     t.integer "geozone_id"
-    t.integer "answer_id"
     t.integer "officer_assignment_id"
     t.integer "user_id"
     t.string "origin"