[Fleet] Remove agent support from Kibana

[nchaulet]

Description

As part of moving to Fleet Server we decided there will be no migration path from 7.12 to 7.13 for existing agents, this mean will remove the ability to enroll/checkin agents from Kibana

Todo:

Fleet:

• [ ] Remove Kibana url settings
• [ ] Remove agents routes
• [x] Remove conditional code wrapped in feature flag checks (PR #97039)

Security Solution:

• [x] Remove artifact download api route (PR #97039)
• [x] remove feature flag code that generate the artifact relative URL (changes from this PR ) (PR #97039)

Need to be done before

Tests and CI:

• [x] elastic-package should be migrated to fleet server https://github.com/elastic/elastic-package/issues/278
• [x] e2e tests should be migrated to fleet server https://github.com/elastic/e2e-testing/issues/438
• [ ] update elastic/upgr8 repo to do upgrades from 7.13.x -> beyond repo: upgr8/issues/1
• [ ] update obs environments to have deploy support for FS, repo: observability-test-environments/issues/1198
• [ ] update Fleet / Endpoint demo / test env to support FS, repo: siem-team/issues/902
• [ ] Endpoint Security (exe) should support download of artifacts from fleet-server (endpoint-dev/issues/8622)

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[paul-tavares]

@nchaulet Ping me (or someone on my team if I am not around) once ready to implement this. you (or I) can open a draft PR and we can then work from the same branch

[EricDavisX]

FYI - there was discussion on this being done and merged this week