Since c66dc591 the PHP Sodium library is required, negating the need
for the OpenSSL equivalent. Remove fallbacks where possible, leaving
only the ability to decrypt legacy OpenSSL-encrypted content (with
debugging).
This isn't really a deprecations where something is replaced
by another and all uses must adapt to. Instead it's just a
fallback functionality that will stop working in Moodle 4.2.
Because of that, I've tried to add TODO/@todo comments to
the places that will need to modified, always pointing to
MDL-71421 that is where the removal will happen.
Also, haven't added any debugging() output, after thinking
a lot about it, because this isn't anything that developers
can be using but a internal implementation detail (a fallback)
that we want to remove in some versions.
