Build: Remove vault/s3 auth for ml artifacts (elastic/x-pack-elasticsearch#3742)
Build: Remove vault/s3 auth for ml artifacts (elastic/x-pack-elasticsearch#3742)

The ML snapshot artifacts bucket is now public, so we no longer need to
grab generated s3 creds from vault. This makes the download task run
noticably faster.

Original commit: elastic/x-pack-elasticsearch@c68a8491c577a59deff5320fa89634f9f1ebee71

strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,vroyer/elassandra,vroyer/elassandra