Delete old_X from codebase

There are some CakeML-preamble-only tactics that were introduced for backward compatibility, e.g.:

https://github.com/CakeML/cakeml/blob/master/misc/preamble.sml#L571

Would be nice to delete them completely from the codebase.

Start with misc/preamble.sml around line 571 and search the codebase for old_X and the CakeML-preamble-only tactics described in the issue. Identify all backward-compatibility definitions and references, remove the obsolete code and confirm the project still builds and its existing checks pass without those references.

Written by the indexing model from the issue text.