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.

compiler/rustc_lint/src/lints.rs

@@ -66,12 +66,6 @@ pub struct BuiltinWhileTrue {
     pub replace: String,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(lint_builtin_box_pointers)]
-pub struct BuiltinBoxPointers<'a> {
-    pub ty: Ty<'a>,
-}
-
 #[derive(LintDiagnostic)]
 #[diag(lint_builtin_non_shorthand_field_patterns)]
 pub struct BuiltinNonShorthandFieldPatterns {