Removes `testenv` annotations and related code. These annotations originally let you skip x-pack snippet tests in the docs. However, that's no longer possible.
Relates to #79309, #31619
Per the [Asciidoctor docs][0], Asciidoctor replaces the following
syntax with double arrows in the rendered HTML:
* => renders as ⇒
* <= renders as ⇐
This escapes several unintended replacements, such as in the Painless
docs.
Where appropriate, it also replaces some double arrow instances with
single arrows for consistency.
[0]: https://asciidoctor.org/docs/user-manual/#replacements
