Remove the box_pointers lint.

As the comment says, this lint "is mostly historical, and not
particularly useful". It's not worth keeping it around.

tests/ui/lint/lint-owned-heap-memory.rs

@@ -1,12 +0,0 @@
-#![allow(dead_code)]
-#![forbid(box_pointers)]
-
-
-struct Foo {
-    x: Box<isize> //~ ERROR type uses owned
-}
-
-fn main() {
-    let _x: Foo = Foo { x : Box::new(10) };
-    //~^ ERROR type uses owned
-}

tests/ui/lint/lint-owned-heap-memory.stderr

@@ -1,20 +0,0 @@
-error: type uses owned (Box type) pointers: Box<isize>
-  --> $DIR/lint-owned-heap-memory.rs:6:5
-   |
-LL |     x: Box<isize>
-   |     ^^^^^^^^^^^^^
-   |
-note: the lint level is defined here
-  --> $DIR/lint-owned-heap-memory.rs:2:11
-   |
-LL | #![forbid(box_pointers)]
-   |           ^^^^^^^^^^^^
-
-error: type uses owned (Box type) pointers: Box<isize>
-  --> $DIR/lint-owned-heap-memory.rs:10:29
-   |
-LL |     let _x: Foo = Foo { x : Box::new(10) };
-   |                             ^^^^^^^^^^^^
-
-error: aborting due to 2 previous errors
-

tests/ui/lint/reasons-erroneous.rs

@@ -9,7 +9,7 @@
 #![warn(bare_trait_objects, reasons = "leaders to no sure land, guides their bearings lost")]
 //~^ ERROR malformed lint attribute
 //~| NOTE bad attribute argument
-#![warn(box_pointers, blerp = "or in league with robbers have reversed the signposts")]
+#![warn(unsafe_code, blerp = "or in league with robbers have reversed the signposts")]
 //~^ ERROR malformed lint attribute
 //~| NOTE bad attribute argument
 #![warn(elided_lifetimes_in_paths, reason("disrespectful to ancestors", "irresponsible to heirs"))]

tests/ui/lint/reasons-erroneous.stderr

@@ -17,10 +17,10 @@ LL | #![warn(bare_trait_objects, reasons = "leaders to no sure land, guides thei
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ bad attribute argument
 
 error[E0452]: malformed lint attribute input
-  --> $DIR/reasons-erroneous.rs:12:23
+  --> $DIR/reasons-erroneous.rs:12:22
    |
-LL | #![warn(box_pointers, blerp = "or in league with robbers have reversed the signposts")]
-   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ bad attribute argument
+LL | #![warn(unsafe_code, blerp = "or in league with robbers have reversed the signposts")]
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ bad attribute argument
 
 error[E0452]: malformed lint attribute input
   --> $DIR/reasons-erroneous.rs:15:36