Summary

Follow up to #246156. It was pointed out in that PR that the Discover customizations example was broken due to changes related to the top nav customization. This PR fixes the issue, but also removes all the unused Discover customizations from the codebase. This is a legacy system we're trying to move away from in favour of context awareness, and they're a maintenance burden that provide no benefit.

The following customizations have been removed, and the only two remaining are the ones currently used by Timelines:

• Top nav
• Data table
• Field list
• Flyout

Checklist

• 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.