remove box_free and replace with drop impl

example/mini_core.rs

@@ -546,7 +546,8 @@ impl<T> Box<T> {
 
 impl<T: ?Sized, A> Drop for Box<T, A> {
     fn drop(&mut self) {
-        // drop is currently performed by compiler.
+        // inner value is dropped by compiler
+        libc::free(self.0.pointer.0 as *mut u8);
     }
 }
 
@@ -563,11 +564,6 @@ unsafe fn allocate(size: usize, _align: usize) -> *mut u8 {
     libc::malloc(size)
 }
 
-#[lang = "box_free"]
-unsafe fn box_free<T: ?Sized>(ptr: Unique<T>, _alloc: ()) {
-    libc::free(ptr.pointer.0 as *mut u8);
-}
-
 #[lang = "drop"]
 pub trait Drop {
     fn drop(&mut self);