Don't create an officer for poll voter factories

For web poll voters, it isn't necessary in order to make the record
valid, and it adds an extra record to the database for each poll
voter created.

spec/factories/polls.rb

@@ -87,7 +87,6 @@ FactoryBot.define do
   factory :poll_voter, class: "Poll::Voter" do
     poll
     association :user, :level_two
-    association :officer, factory: :poll_officer
     from_web
 
     trait :from_web do
@@ -103,7 +102,9 @@ FactoryBot.define do
       end
 
       officer_assignment do
-        association :poll_officer_assignment, booth_assignment: booth_assignment, officer: officer
+        association :poll_officer_assignment,
+          booth_assignment: booth_assignment,
+          officer: officer || association(:poll_officer)
       end
     end