Remove special overriding logic for explicit nulls

[HarrisL2]

Fixes #22071

[olhotak]

The changes LGTM, but we'll need to get the CI running first.

There we see:

object creation impossible, since def compareTo(x$0: T | Null): Int in trait Comparable in package java.lang is not defined

The type of x should become a flexible type, not T | Null. So the question is why doesn't it become a flexible type.