Remove dead InternalBufWriter implementation

In 8d90d3f `BufStream`, the only
consumer of `InternalBufWriter`, was removed. As implied by the name,
this type is private, hence it is currently dead code.

src/libstd/io/buffered.rs

@@ -772,21 +772,6 @@ impl<W: Write> fmt::Debug for LineWriter<W> where W: fmt::Debug {
     }
 }
 
-struct InternalBufWriter<W: Write>(BufWriter<W>);
-
-impl<W: Read + Write> InternalBufWriter<W> {
-    fn get_mut(&mut self) -> &mut BufWriter<W> {
-        let InternalBufWriter(ref mut w) = *self;
-        return w;
-    }
-}
-
-impl<W: Read + Write> Read for InternalBufWriter<W> {
-    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
-        self.get_mut().inner.as_mut().unwrap().read(buf)
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use prelude::v1::*;