Cleanup user namespace transition code

By the time we get to this MR, we should have merged #341069 (closed), which means all entries in the namespace table is either Group, Project, or User

Checking for nil and Namespaces::UserNamespace.sti_name was only a temporary step to moving completely to Namespaces::UserNamespace. Re-evaluate code that was added in !68894 (merged) or elsewhere to handle the duplicate path.

Remove namespace code relying on type == nil

• Remove namespace code relying on type == nil (!73152 (merged))
• Search code for https://gitlab.com/gitlab-org/gitlab/-/issues/341070 for TODOs that indicate how code should be updated (!73493 (merged))