configure: Remove help for --disable-vnc-thread, --enable-vnc-thread

Commit 2624bab8 removed these
configure arguments. Now the help text for both is removed, too.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -1043,8 +1043,6 @@ echo "  --disable-vnc-jpeg       disable JPEG lossy compression for VNC server"
 echo "  --enable-vnc-jpeg        enable JPEG lossy compression for VNC server"
 echo "  --disable-vnc-png        disable PNG compression for VNC server (default)"
 echo "  --enable-vnc-png         enable PNG compression for VNC server"
-echo "  --disable-vnc-thread     disable threaded VNC server"
-echo "  --enable-vnc-thread      enable threaded VNC server"
 echo "  --disable-curses         disable curses output"
 echo "  --enable-curses          enable curses output"
 echo "  --disable-curl           disable curl connectivity"