Remove is_null
It was confusingly named (`is_zero` would have been better, as someone pointed out somewhere but I forgot who or where), and it didn't even reliably test for "is this value 0 at run-time" because out-of-bounds pointers *can* be 0.
It's not used in rustc, and miri only really needs `is_null_ptr` and `to_bytes() == 0`, so let's just kill this method.
r? @oli-obk