[Code] Remove code_admin/code_user roles

[mw-ding]

We currently use the Kibana Feature Control to manage user access control. And thus the roles in elasticsearch is not required anymore.

Addressing https://github.com/elastic/code/issues/1659

[tvernum]

@mw-ding Elasticsearch PRs need to be labelled with:

• one or more version labels (starting with v) that indicate which versions this change will target
• one area label (starting with :) that indicate which functional area this change affects
• one change-type label (starting with >) to control how it should be shown in the release notes.

I can add the last 2, but can you please add version labels, thanks.

[elasticmachine]

Pinging @elastic/es-security (:Security/Authorization)

[mw-ding]

@tvernum Done, please take a look. Also, do you think the failed tests here are relevant?

[tvernum]

@mw-ding This failure https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+pull-request-1/9039/testReport/junit/org.elasticsearch.client.documentation/SecurityDocumentationIT/testGetRoles/ is definitely related.
Can you update that test to reflect the removal of these roles?

The other one doesn't appear to be.

[mw-ding]

@tvernum the test has been updated. There is still a packaging-sample-matrix test failing though.