Do not check for S3 blob to exist before writing

[ywelsch]

Amazon S3 provides a weak consistency model (see https://docs.aws.amazon.com/AmazonS3/latest/dev/Introduction.html). From the docs (emphasis mine):

Amazon S3 provides read-after-write consistency for PUTS of new objects in your S3 bucket in all regions with one caveat. The caveat is that if you make a HEAD or GET request to the key name (to find if the object exists) before creating the object, Amazon S3 provides eventual consistency for read-after-write.
Amazon S3 offers eventual consistency for overwrite PUTS and DELETES in all regions.
Updates to a single key are atomic. For example, if you PUT to an existing key, a subsequent read might return the old data or the updated data, but it will never write corrupted or partial data.

In #19749 an extra check was added before writing each blob to ensure that we would not be overriding an existing blob. Due to S3's weak consistency model, this check was best effort. To make matters worse, however, this resulted in a HEAD request to be done before every PUT, in particular also when PUTTING a new object. The approach taken in #19749 worsened our consistency guarantees for follow-up snapshot actions, as it made it less likely for new files that had been written to be available for reads.

My suggestion is therefore to remove this extra check. Due to the weak consistency model, this check was a best effort thing anyway, and there's currently no way to prevent accidental overrides on S3.

[elasticmachine]

Pinging @elastic/es-distributed

[jasontedor]

LGTM.

[tlrx]

LGTM. Thanks @ywelsch, I like these improvements.