Clean up patient creation in session management

This was creating an extra session, location, etc, which would sometimes
trigger an error if the extra location created had the same name as the
one being used in the test. Rare, but still annoying.

spec/features/session_management_spec.rb

@@ -47,8 +47,7 @@ def given_my_team_is_running_an_hpv_vaccination_campaign
     @team = create(:team, :with_one_nurse, :with_one_location)
     create(:campaign, :hpv, team: @team)
     @location = @team.locations.first
-    @patient = create(:patient)
-    @location.patients << @patient
+    @patient = create(:patient, location: @location, session: nil)
   end
 
   def when_i_go_to_todays_sessions_as_a_nurse