Remove the 'custom subject suffix' spec from a shared example

This one-off test case is covered in the Notify mailer spec, so there
was no benefit in testing it every time this shared example was
referenced.

spec/support/notify_shared_examples.rb

@@ -37,16 +37,6 @@ shared_examples 'an email sent from GitLab' do
     reply_to = subject.header[:reply_to].addresses
     expect(reply_to).to eq([gitlab_sender_reply_to])
   end
-
-  context 'when custom suffix for email subject is set' do
-    before do
-      stub_config_setting(email_subject_suffix: 'A Nice Suffix')
-    end
-
-    it 'ends the subject with the suffix' do
-      is_expected.to have_subject /\ \| A Nice Suffix$/
-    end
-  end
 end
 shared_examples 'an email that contains a header with author username' do