Remove redundant setting resets in after blocks

Settings are stored in the database, and so any changes to the settings
done during the tests are automatically rolled back between one test and
the next one.

There were also a few places where we weren't using an `after` block but
changing the setting at the end of the test.

spec/helpers/signature_sheets_helper_spec.rb

@@ -17,10 +17,6 @@ describe SignatureSheetsHelper do
       Setting["feature.remote_census"] = true
     end
 
-    after do
-      Setting["feature.remote_census"] = nil
-    end
-
     it "returns text help when date_of_birth and postal_code are not required" do
       text_help_1 = "To verify a user, your application needs: Document number"
       text_help_2 = "Required fields for each user must be separated by commas and each user must be separated by semicolons."