Remove files[] directive from Mailer.info

As per When to use files directive in module's info file, only files with classes should be included in .info files[] directive. hook_init() is already loading it in the .module, if needed:

 function user_relationship_mailer_init() {
  module_load_include('inc', 'user_relationship_mailer', 'user_relationship_mailer_defaults');
}