Refactor: cleanup unused (old) method

[kares]

• cleanup unused (old) methods was used in V1 - no longer relevant with aws-sdk 2.x (explanation at https://github.com/logstash-plugins/logstash-output-s3/pull/108#issuecomment-582339375) (since its private it never got to be called from the mixin)

with https://github.com/logstash-plugins/logstash-mixin-aws/pull/42 this caused some aws gem newbie confusion ...

[kares]

please also review: https://github.com/logstash-plugins/logstash-mixin-aws/pull/42 who ever lands here ...

[andsel]

LGTM