remove redundant code

library/alloc/src/vec.rs

@@ -2386,13 +2386,7 @@ 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();
-            let dst_slice = slice::from_raw_parts_mut(self.as_mut_ptr().add(len), slice.len());
-            dst_slice.copy_from_slice(slice);
-            self.set_len(len + slice.len());
-        }
+        unsafe { self.append_elements(slice) };
     }
 }