remove Scalar::is_null_ptr
remove Scalar::is_null_ptr

Miri does not need it any more and it is otherwise unused

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust