[DOCS] Remove `ifdef::asciidoctor[]` conditionals after Asciidoctor migration

To support both AsciiDoc and Asciidoctor during Asciidoctor migration, the `ifdef::asciidoctor[]` conditional was sometimes used. See elastic/elasticsearch#41615 for an example.

After the Asciidoctor migration is complete, these conditionals will no longer be needed. We should remove them then.

Relates to elastic/docs#827.
