buffer.h: remove list::iterator_impl::advance(size_t)

quote from Radoslaw's comment

> In general we could remove it as the `buffer` library tends to
> disrespects `size_t` – `list::_len` and `ptr::_len` are both
`unsigned`.
> There are some exceptions like `size_t buffer::ptr::get_offset()`
(used
> altogether with `::advance` in `denc.h`) but they deserve rework as
> well.

and in this very case, we simply cast `size_t` to `unsigned`. so we
could silently trim a 64bits integer to 32bit on 64bits architecture.

so for better portability, we can just remove it. also, this function is
inlined, so the ABI is not changed.

Signed-off-by: Kefu Chai <kchai@redhat.com>

src/include/buffer.h

@@ -727,9 +727,7 @@ inline namespace v14_2_0 {
 	//return off == bl->length();
       }
 
-      void advance(int o) = delete;
       void advance(unsigned o);
-      void advance(size_t o) { advance(static_cast<unsigned>(o)); }
       void seek(unsigned o);
       char operator*() const;
       iterator_impl& operator++();