Remove support for non nested filtering

CONFLUENCE-196 in version 9.35.0 introduced nested space filtering, removing the need to use the nested page migrator application to fix the nesting after the import.

Now that direct nested filtering has been stabilized and we know it works well, it should be the default mode.

Also, given that:

• link and permission handling is a bit broken in non nested filtering, and that's not something we can really fix
• having to support the two modes makes the code more complex and difficult to maintain

We shall deprecate, ignore and hide (with @PropertyHidden) the nestedSpace property, making the nested filtering the only mode and simplifying the code a lot.

Feature removal is not technically an improvement, I'm thus marking this ticket as "Task", though removing the extra branching can only make the code faster, if it does.