Remove feature flag for ES web indexing

What does this MR do?

This removes the feature flag for the ES web indexing button (#12712 (closed)).

We're also tweaking the migration db/migrate/20190916000001_fill_elasticsearch_indices.rb from !15275 (merged) (not in master yet) to only run in EE, because it was failing now in gitlab-foss. At the time of the previous MR we didn't test FOSS on CI yet, and the migration itself was already reviewed there as well.

This MR will be merged into es-admin-step2 / !17230 (closed) which acts as our temporary integration branch, until we're ready to merge something usable into master.

For this reason this MR also doesn't add a changelog entry, because the current ES web indexing button is being replaced by a completely new UI in !18254 (merged).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers