Remove SELF_CONTAINED as a Catalog type

catalog_type distinguishes between ABSOLUTE_PUBLISHED, RELATIVE_PUBLISHED, and SELF_CONTAINED.

However the only different between RELATIVE_PUBLISHED and SELF_CONTAINED is that there is no self link on the root catalog of a SELF_CONTAINED catalog.

I don't think this is enough of a difference to warrant calling out as a catalog type. For one, the absolute self link at root isn't really used in most (all?) tooling other than for user to know where the root cat lives. I guess stac-browser uses it to display and create the absolute link for it.

You can always just ignore the self link or remove it entirely. Or we could just have an option to save it or now when you save a catalog (include_self_on_root). This is clearer than having a catalog type where it isn't obvious what the practical difference is.

Start by tracing the catalog_type entry point and every use of ABSOLUTE_PUBLISHED, RELATIVE_PUBLISHED, and SELF_CONTAINED. Determine whether the intended outcome is removing SELF_CONTAINED or adding an include_self_on_root save option; the issue does not define acceptance criteria, so completion would require a maintainer decision.