Remove authorized_only param from issuable finder

What does this MR do and why?

This has not been used for a very long time. I traced this back to this commit: 2f69213e.

I don't see this param being passed anywhere and we don't have a use case for returning only issues from authorized projects and excluding public projects the user has access to.

There's reference of this in https://docs.gitlab.com/ee/development/reusing_abstractions.html#why-these-guidelines-are-necessary but I left it as-is since that's meant to be a code example only and isn't up to date with the current code.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.