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 Remove help message which is now no longer valid.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>