docs: core-api: Drop reference to flexible-arrays

This file doesn't exist anymore.

Fixes: 586187d7 ("Drop flex_arrays")
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>