Don't check the database for voters in system tests

As mentioned in commits like a586ba806, a7664ad81, 006128da5, b41fbfa52
and c480cdd91, accessing the database after starting the browser with
the `visit` method sometimes results in database corruption and failing
tests on our CI due to the process running the test accessing the
database after the process running the browser has started.

In these cases, there's no need to check the database; we're already
checking the application behavior that shows that the voters have been
correctly created.

spec/support/common_actions/polls.rb

@@ -18,8 +18,6 @@ module Polls
 
     first(:button, "Confirm vote").click
     expect(page).to have_content "Vote introduced!"
-
-    expect(Poll::Voter.count).to eq(1)
   end
 
   def confirm_phone(code:)

spec/system/budget_polls/voter_spec.rb

@@ -29,9 +29,6 @@ describe "BudgetPolls", :with_frozen_time do
         expect(page).to have_content "Vote introduced!"
       end
 
-      expect(Poll::Voter.count).to eq(1)
-      expect(Poll::Voter.first.origin).to eq("booth")
-
       logout
       login_as(admin.user)
       visit admin_poll_recounts_path(poll)

spec/system/polls/voter_spec.rb

@@ -87,9 +87,6 @@ describe "Voter" do
         expect(page).to have_content("Vote introduced!")
       end
 
-      expect(Poll::Voter.count).to eq(1)
-      expect(Poll::Voter.first.origin).to eq("booth")
-
       logout
       login_as(admin.user)
       visit admin_poll_recounts_path(poll)
@@ -145,7 +142,6 @@ describe "Voter" do
       scenario "Trying to vote in web and then in booth" do
         login_as user
         vote_for_poll_via_web(poll, question, "Yes")
-        expect(Poll::Voter.count).to eq(1)
 
         logout
         login_through_form_as_officer(officer)
@@ -172,7 +168,6 @@ describe "Voter" do
         end
         expect(page).to have_content "You have already participated in a physical booth. " \
                                      "You can not participate again."
-        expect(Poll::Voter.count).to eq(1)
 
         logout
         login_as(admin.user)
@@ -211,7 +206,6 @@ describe "Voter" do
 
       expect(page).to have_content "You have already participated in a physical booth. " \
                                    "You can not participate again."
-      expect(Poll::Voter.count).to eq(1)
 
       logout
       login_as(admin.user)