remove redundant code

Author: the8472

src/liballoc/vec.rs

@@ -2281,12 +2281,7 @@ where
 {
     fn spec_extend(&mut self, iterator: slice::Iter<'a, T>) {
         let slice = iterator.as_slice();
-        self.reserve(slice.len());
-        unsafe {
-            let len = self.len();
-            self.set_len(len + slice.len());
-            self.get_unchecked_mut(len..).copy_from_slice(slice);
-        }
+        unsafe { self.append_elements(slice) };
     }
 }