chgserver: remove Python 2 file descriptor logic

Follows up 0bb28b7736bc "chgserver: remove Python 2 support code."

On Python 2, we had to close newfp prior to restoring the original file
description since "delete newfp" would otherwise close the file descriptor
shared with the long-lived fp:

  in attachio():
    newfp = os.fdopen(fp.fileno(), mode, bufsize)
  in _restoreio():
    newfp.close()  # temporarily close newfp.fileno() (= fp.fileno())
    os.dup2(fd, fp.fileno())  # reopen fp.fileno() with original fd

On the other hand, we shouldn't call newfp.close() on Python 3 since
any function calls are proxied to the underlying file object by
procutil.LineBufferedWrapper.

mercurial/chgserver.py

@@ -438,14 +438,8 b' class chgcmdserver(commandserver.server)'
         nullfd = os.open(os.devnull, os.O_WRONLY)
         ui = self.ui
         for (ch, fp, fd), (cn, fn, mode) in zip(self._oldios, _iochannels):
-            newfp = getattr(ui, fn)
-            # On Python 3, newfp is just a wrapper around fp even if newfp is
-            # not fp, so deleting newfp is safe.
-            if newfp is not fp:
-                newfp.close()
-            # restore original fd: fp is open again
             try:
-                if newfp is fp and 'w' in mode:
+                if 'w' in mode:
                     # Discard buffered data which couldn't be flushed because
                     # of EPIPE. The data should belong to the current session
                     # and should never persist.