Hide subdomains if there aren't any defined

This is the case at the moment, because the domain model we've pulled in
from CaDeT doesn't have subdomains. This might change later though so I
don't want to remove the subdomain code completely.

templates/partial/filter.html

@@ -23,10 +23,12 @@ <h2 class="govuk-heading-m">Filter</h2>
               <label class="govuk-label" for="{{form.domain.id_for_label}}">Top-level domain</label>
               {{form.domain}}
             </div>
+            {% if form.subdomain.choices %}
             <div class="govuk-form-group js-required">
               <label class="govuk-label" for="{{form.subdomain.id_for_label}}">Subdomain</label>
               {{form.subdomain}}
             </div>
+            {% endif %}
           </fieldset>
         </div>
         <div class="govuk-form-group">

tests/selenium/test_search_scenarios.py

@@ -147,7 +147,6 @@ def test_clear_single_filter(self):
         self.verify_domain_selected(domain)
         self.click_clear_selected_filter(domain)
         self.verify_unselected_domain()
-        self.verify_unselected_subdomain()
 
     def test_clear_all_filters(self):
         """
@@ -271,9 +270,6 @@ def enter_a_query_and_submit(self, query):
     def select_domain(self, domain):
         self.search_page.select_domain(domain)
 
-    def select_subdomain(self, domain):
-        self.search_page.select_subdomain(domain)
-
     def click_option(self, sortby):
         self.search_page.sort_label(sortby).click()
 
@@ -295,18 +291,10 @@ def verify_domain_selected(self, domain):
         selected_domain = self.search_page.get_selected_domain().text
         assert selected_domain == domain
 
-    def verify_subdomain_selected(self, domain):
-        selected_domain = self.search_page.get_selected_subdomain().text
-        assert selected_domain == domain
-
     def verify_unselected_domain(self):
         selected_domain = self.search_page.get_selected_domain().text
         assert selected_domain == "All domains"
 
-    def verify_unselected_subdomain(self):
-        selected_domain = self.search_page.get_selected_subdomain().text
-        assert selected_domain == "All subdomains"
-
     def verify_selected_filters_shown(self, domains):
         actual = {i.text for i in self.search_page.selected_filter_tags()}
         expected = set(domains)