[Fleet] Remove preconfiguration API route

[nchaulet]

Summary

Resolve #146177

Remove the /api/fleet/setup/preconfiguration API that route was used to test preconfiguration, but it should not be used by end user as the source of truth for preconfigured agent policies is the kibana.yml file and using that API can cause some bugs.

Release note

Remove the /api/fleet/setup/preconfiguration API, that route was made public by accident and not should be used by end user as the only way to manage preconfigured agent policy is through kibana config file.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[nchaulet]

@elasticmachine merge upstream

[criamico]

LGTM

[kpollich]

This should probably get a release note label on the off chance a user was relying on this API.

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 2886273

Metrics [docs]

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
fleet	120.5KB	120.5KB	-51.0B

Unknown metric groups

ESLint disabled in files

id	before	after	diff
osquery	1	2	+1

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	19	21	+2
fleet	59	65	+6
osquery	109	115	+6
securitySolution	445	451	+6
total			+20

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	20	22	+2
fleet	68	74	+6
osquery	110	117	+7
securitySolution	521	527	+6
total			+21

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @nchaulet