Remove unused return value of admin login helper

Well, there was one use of the return helper. But I don't think that
it's a very intuitive method.

spec/support/request/authentication_helper.rb

@@ -4,9 +4,7 @@ module AuthenticationHelper
   include Warden::Test::Helpers
 
   def login_as_admin
-    admin_user = create(:admin_user)
-    login_as admin_user
-    admin_user
+    login_as create(:admin_user)
   end
 
   def login_as_admin_and_visit(path_visit)

spec/system/admin/enterprises_spec.rb

@@ -22,6 +22,7 @@ describe '
   end
 
   it "creating a new enterprise" do
+    admin = create(:admin_user)
     eg1 = create(:enterprise_group, name: 'eg1')
     eg2 = create(:enterprise_group, name: 'eg2')
     payment_method = create(:payment_method)
@@ -29,7 +30,7 @@ describe '
     enterprise_fee = create(:enterprise_fee)
 
     # Navigating
-    admin = login_as_admin
+    login_as admin
     visit '/admin/enterprises'
     click_link 'New Enterprise'