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

Commit 12d4536f 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Remove help message which is now no longer valid.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: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -1025,7 +1025,6 @@ echo "  --disable-linux-aio      disable Linux AIO support"
 echo "  --enable-linux-aio       enable Linux AIO support"
 echo "  --disable-attr           disables attr and xattr support"
 echo "  --enable-attr            enable attr and xattr support"
-echo "  --enable-io-thread       enable IO thread"
 echo "  --disable-blobs          disable installing provided firmware blobs"
 echo "  --enable-docs            enable documentation build"
 echo "  --disable-docs           disable documentation build"