Drop the insertion order option

Now that we're v3-only, there's very little value in providing the old random approach with its gotchas. There will inevitably be a few users who prefer the way they're used to, but I don't think we can justify keep things confusing for the typical user for the benefit of a few.

I propose we change Anki so that sequential is forced, and the option is removed from the GUI. The internal setting would remain, so it continues to sync/persist when changed by older clients.

Originally reported on https://forums.ankiweb.net/t/confusing-message-about-sequential-due-number-on-v3/51668/3

No files, tests, or entry points are named. Start by locating the insertion-order option in Anki's GUI and tracing its internal setting; done means sequential insertion is forced and the GUI option is removed without breaking sync or persistence for older clients.

Written by the indexing model from the issue text.