Fix RSpec/InferredSpecType cop (#24736)

spec/mailers/admin_mailer_spec.rb

@@ -2,7 +2,7 @@
 
 require 'rails_helper'
 
-RSpec.describe AdminMailer, type: :mailer do
+RSpec.describe AdminMailer do
   describe '.new_report' do
     let(:sender)    { Fabricate(:account, username: 'John') }
     let(:recipient) { Fabricate(:account, username: 'Mike') }

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') }

spec/mailers/user_mailer_spec.rb

@@ -2,7 +2,7 @@
 
 require 'rails_helper'
 
-describe UserMailer, type: :mailer do
+describe UserMailer do
   let(:receiver) { Fabricate(:user) }
 
   shared_examples 'localized subject' do |*args, **kwrest|