Remove redundant rubocop disable directive in test/helper.rb

The Style/OneClassPerFile disable comment was never needed since the
file doesn't define multiple top-level classes. Rubocop now flags it
as Lint/RedundantCopDisableDirective.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: petergoldstein

test/helper.rb

@@ -33,7 +33,6 @@ def warn(msg, **, &)
 # Generate self-signed certs for SSL once per suite run.
 CertificateGenerator.generate
 
-# rubocop:disable Style/OneClassPerFile
 module Minitest
   class Spec
     include Memcached::Helper
@@ -77,4 +76,3 @@ def with_nil_logger
     end
   end
 end
-# rubocop:enable Style/OneClassPerFile