[UII] Remove multi-page add integration flow (?useMultiPageLayout)

[jen-huang]

Summary

Resolves #271315. Does what it says on the tin: removes the multi-page add integration flow for first time Elastic Agent users (previously triggered using the query param in /add-integration?useMultiPageLayout).

Security solution has a custom view registered in this flow. That has been removed, as well as the extension point.

This will get backported to all open branches.

Checklist

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

• Unit or functional tests were updated or added to match the most common scenarios
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines
• Review the backport guidelines and apply applicable backport:* labels.

[infra-vault-gh-plugin-prod]

Pinging @elastic/fleet (Team:Fleet)

[criamico]

LGTM 🚀

[szwarckonrad]

DW changes LGTM!

[jen-huang]

@elasticmachine merge upstream

[kibanamachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 5d7097b
• Build duration: 62 mins

Failed CI Steps

• FTR Configs #61
• FTR Configs #131
• Scout Lane #9 - stateful-classic / default
• Scout Lane #11 - stateful-classic / default
• Serverless Detection Engine - Exceptions - Security Solution Cypress Tests #1

Test Failures

• [job] [logs] FTR Configs #131 / Cloud Security Posture - Group 5 (KSPM + Flyouts) Security Alerts Page - Graph visualization expanded flyout - filter by node
• [job] [logs] Scout Lane #9 - stateful-classic / default / local-stateful-classic - Create Data View wizard - data stream is accepted as an index pattern source and wizard auto-detects the timestamp field
• [job] [logs] Scout Lane #11 - stateful-classic / default / local-stateful-classic - UptimeIntegrationDeprecation - returns true when non-managed synthetics policies exist

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
fleet	2236	2215	-21
securitySolution	9585	9583	-2
total			-23

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
fleet	2.8MB	2.8MB	-33.4KB
securitySolution	12.1MB	12.1MB	-2.7KB
total			-36.1KB

Page load bundle

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

id	before	after	diff
fleet	212.0KB	211.8KB	-199.0B
securitySolution	159.8KB	159.6KB	-233.0B
total			-432.0B

Unknown metric groups

async chunk count

id	before	after	diff
securitySolution	119	118	-1

ESLint disabled line counts

id	before	after	diff
fleet	43	37	-6

Total ESLint disabled count

id	before	after	diff
fleet	51	45	-6

History

• 💔 Build #450531 failed bd9d6ab
• 💛 Build #450000 was flaky 132efc4
• 💔 Build #449965 failed 6de3913

cc @jen-huang