This commit removes the "legacy" feature of secure settings, which setup
a parallel setting that was a fallback in the insecure
elasticsearch.yml. This was previously used to allow the new secure
setting name to be that of the old setting name, but is now not in use
due to other refactorings. It is much cleaner to just have all secure
settings use new setting names. If in the future we want to reuse the
previous setting name, once support for the insecure settings have been
removed, we can then rename the secure setting. This also adds a test
for the behavior.
73b8aad9a3