Remove warning header for consistency

This isn't a standard header, and the other docs don't use it, so let's remove it.

Author: steveklabnik

src/libstd/io/buffered.rs

@@ -407,8 +407,6 @@ impl<W: Write> BufWriter<W> {
 
     /// Gets a mutable reference to the underlying writer.
     ///
-    /// # Warning
-    ///
     /// It is inadvisable to directly write to the underlying writer.
     ///
     /// # Examples
@@ -835,8 +833,6 @@ impl<S: Read + Write> BufStream<S> {
 
     /// Gets a mutable reference to the underlying stream.
     ///
-    /// # Warning
-    ///
     /// It is inadvisable to read directly from or write directly to the
     /// underlying stream.
     pub fn get_mut(&mut self) -> &mut S {