programmatic access test objects litter bucket

[gaul]

After #85 merged, my bucket has several test objects litter:

$ aws s3 ls s3://my-bucket
2016-09-06 12:11:49          4 logstash-programmatic-access-test-object-1473189107
2016-09-06 12:20:18          4 logstash-programmatic-access-test-object-1473189616

This functionality was removed due to issues with access policies. Instead of failing like before or littering like now, the plugin could attempt to remove the objects and ignore failures.