Fix an easy to trigger deadlock in std::io::stdio

Being a person who somehow has taken a liking to premature optimisation, my knee-jerk reaction to
locking in std handles was preamble resembling following snippet:

    let stdout = stdout();
    let lstdout = stdout.lock();
    let stdin = stdin();
    let lstdin = stdin.lock();

and then reading from the locked handle like this:

    let mut letter = [0; 1];
    lstdin.read(&mut letter).unwrap();

As it is now this code will deadlock because the `read` method attempts to lock stdout as well!

src/libstd/io/stdio.rs

@@ -157,9 +157,6 @@ impl Read for Stdin {
 
 impl<'a> Read for StdinLock<'a> {
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
-        // Flush stdout so that weird issues like a print!'d prompt not being
-        // shown until after the user hits enter.
-        drop(stdout().flush());
         self.inner.read(buf)
     }
 }