Remove incorrect comment in Vec::drain

Author: WaffleLapkin

library/alloc/src/vec/mod.rs

@@ -1999,9 +1999,7 @@ impl<T, A: Allocator> Vec<T, A> {
         unsafe {
             // set self.vec length's to start, to be safe in case Drain is leaked
             self.set_len(start);
-            // Use the borrow in the IterMut to indicate borrowing behavior of the
-            // whole Drain iterator (like &mut T).
-            let range_slice = slice::from_raw_parts_mut(self.as_mut_ptr().add(start), end - start);
+            let range_slice = slice::from_raw_parts(self.as_ptr().add(start), end - start);
             Drain {
                 tail_start: end,
                 tail_len: len - end,