Remove obsolete XXX comment about non-blocking I/O

The comment suggested rewriting Popen._communicate() to use
non-blocking I/O on file objects now that Python 3's io module
is used instead of C stdio.

This is unnecessary - the current approach using select() to
detect ready fds followed by os.read()/os.write() is correct
and efficient. The selector already solves "when is data ready?"
so non-blocking mode would add complexity with no benefit.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: gpshead

Lib/subprocess.py

@@ -2655,9 +2655,6 @@ def _communicate(self, input, endtime, orig_timeout):
                     ready = selector.select(timeout)
                     self._check_timeout(endtime, orig_timeout, stdout, stderr)
 
-                    # XXX Rewrite these to use non-blocking I/O on the file
-                    # objects; they are no longer using C stdio!
-
                     for key, events in ready:
                         if key.fileobj is self.stdin:
                             chunk = input_view[self._input_offset :