[Search] Remove unused search-kibana code

[seialkali]

Summary

This PR removes unused code and files across the following search-kibana plugins and packages:

• enterprise_search
• content_connectors

Checklist

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support`
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• 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.

[elasticmachine]

💔 Build Failed

• Buildkite Build
• Commit: 71a4451

Failed CI Steps

• Build Kibana Distribution
• Linting
• Check Types

History

• 💔 Build #344528 failed 632963b