Remove the old strictNullChecks implementation

[k163377]

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.