Remove the old `strictNullChecks` implementation

`strictNullChecks` will be migrated to a new, more performant and maintainable backend.
On the one hand, this migration will be phased in for `2.x`, as the migration involves disruptive changes.

However, since the `3.x` release is earlier than the final removal in `2.x`, the implementation related to the old option will be removed ahead of time in `3.x`.