remove Scalar::is_null_ptr

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

src/librustc/mir/interpret/value.rs

@@ -275,17 +275,6 @@ impl<'tcx, Tag> Scalar<Tag> {
         }
     }
 
-    #[inline]
-    pub fn is_null_ptr(self, cx: &impl HasDataLayout) -> bool {
-        match self {
-            Scalar::Raw { data, size } => {
-                assert_eq!(size as u64, cx.data_layout().pointer_size.bytes());
-                data == 0
-            },
-            Scalar::Ptr(_) => false,
-        }
-    }
-
     #[inline]
     pub fn from_bool(b: bool) -> Self {
         Scalar::Raw { data: b as u128, size: 1 }