configure: Remove relicts from --enable-io-thread

Commit 12d4536f7d911b6d87a766ad7300482ea663cea2 removed
configure option --enable-io-thread.

Remove help message which is now no longer valid.

Cc: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
