Fix RSpec/InferredSpecType cop (#24736)

spec/mailers/notification_mailer_spec.rb

@@ -2,7 +2,7 @@
 
 require 'rails_helper'
 
-RSpec.describe NotificationMailer, type: :mailer do
+RSpec.describe NotificationMailer do
   let(:receiver)       { Fabricate(:user) }
   let(:sender)         { Fabricate(:account, username: 'bob') }
   let(:foreign_status) { Fabricate(:status, account: sender, text: 'The body of the foreign status') }