[ML] Remove the undocumented "delimited" format for post_data

[droberts195]

The data_description of anomaly detection jobs used to accept
delimited data, although this was never documented.

This change removes the delimited option from the data_description,
and the associated functionality in post_data that handled it.

This is not a breaking change because it's removing functionality
that officially never existed. However, just in case somebody
was using it it is only removed from 8.0 and higher, so that at
least they won't find out during a patch install.

[elasticmachine]

Pinging @elastic/ml-core (Team:ML)

[benwtrent]

I love seeing all the code go.

I just want to triple check: Users who use POST data have been getting deprecation warnings about the CSV format in 7.x. So, they are prepared for this change. Yeah, its undocumented, but folks could be doing all sorts of things.

[droberts195]

I just want to triple check: Users who use POST data have been getting deprecation warnings about the CSV format in 7.x. So, they are prepared for this change. Yeah, its undocumented, but folks could be doing all sorts of things.

The deprecation warning has been here since 5.4:

elasticsearch/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/job/JobManager.java

Lines 277 to 280 in 2350369

	if (job.getDataDescription() != null && job.getDataDescription().getFormat() == DataDescription.DataFormat.DELIMITED) {
	deprecationLogger.deprecate(DeprecationCategory.API, "ml_create_job_delimited_data",
	"Creating jobs with delimited data format is deprecated. Please use xcontent instead.");
	}

So they were warned when they first created the job. The UI has never provided a way to create jobs that take delimited data, and it's never been documented.

If this was an experimental feature we'd be allowed to remove it. I think an undocumented feature carries even fewer guarantees than an experimental feature.

The other thing is, if somebody had been using a job by posting delimited data to it, we don't delete the job or its results on upgrade. They could continue to use it by switching to posting JSON data instead.

[elasticmachine]

Pinging @elastic/clients-team (Team:Clients)

[droberts195]

@elastic/clients-team the change needed here is to remove field_delimiter from the DataDescription object, i.e. this line: https://github.com/elastic/elasticsearch-specification/blob/8b0e9ed594e3f10c1e36836222a3d3f0d87ed9f0/specification/ml/_types/Job.ts#L127

field_delimiter has never officially existed, but since the current clients include it it should probably remain available in the clients until 7.last then be removed in 8.0 to match what this PR does. quote_character (also undocumented from the start) never made it into the clients as it was never mentioned in a YAML test. I think it's best not to add that to 7.x.