Cleanup: Remove unused/underused API from the beats feature package

[faec]

This is entirely an internal cleanup, it should have no functional change.

Most of the code in libbeat/feature is unused. That package defines a global registry of plugins and features that lets callers register arbitrary factories that can be queried under appropriate namespaces. It was originally intended as an interim step in library migration (see the somewhat poignant 5-year-old comment // Registry is the global plugin registry, this variable is meant to be temporary...), but many of its features were never used and after some recent cleanups its only remaining client is Functionbeat (which still only uses a small part of its functionality, relying mostly on functionbeat-specific extensions in x-pack/functionbeat/function/provider).

This PR removes the unused parts of the package and clarifies/simplifies the code that is still used. A reasonable followup might be to move the remaining API entirely into Functionbeat, which has unique use cases for it that have grown beyond the original package, however I wanted to do an in-place cleanup first to simplify code review.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @faec? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2023-03-30T11:27:42.422+0000

• Duration: 85 min 53 sec

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