[Fleet] Remove experimental indexing toggles

[kpollich]

Ref #132818

In elastic/ingest-docs#151, we're moving towards providing an API-first approach to toggling experimental data stream features like synthetic source, TSDB, etc.

Currently, these toggles are gated by a feature flag, and are only used internally for testing and experimentation. With the advent of direct package upload elastic/elastic-package#1084 for better integrations DX, these toggles are not necessary. So, we're opting to remove them entirely.

Implementation

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1. Remove all code for rendering experimental indexing feature toggles UI
2. Remove feature flag entirely

Blocked by elastic/ingest-docs#151. We need to make sure we have public docs available for disabling these features in case users (internal or otherwise) run into problems with this functionality.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[jillguyonnet]

I'm unsure about the third requirement:

Remove feature flag from ECE Kibana user settings allowlist

I see that the xpack.fleet.enableExperimental setting is allowlisted in cloud, but I'm not finding a predefined list of features.

@juliaElastic If you have a moment, do you know the answer to this question?

[jlind23]

Maybe @nchaulet knows as he is the one who pushed: https://github.com/elastic/cloud/pull/111041

[nchaulet]

Actually I am not sure we should remove the flag from ECE as it's not specific to that feature, probably better to keep it in the allowlist

[jillguyonnet]

So was my understanding correct that it's the experimental features setting that is allowlisted, not the specific feature flags?

[juliaElastic]

enableExperimental includes all Fleet feature flags, not just the experimental indexing feature, so we should leave it in ECE I think.

[jlind23]

As stated by @juliaElastic and @nchaulet over slack, it doesn't make sense to remove the ECE allowlist as the enableExperimental setting includes all Fleet feature flags:

kibana/x-pack/plugins/fleet/common/experimental_features.ts

Line 11 in 8389288

* A list of allowed values that can be used in `xpack.fleet.enableExperimental`.

Thus, closing this issue as done.