QEMU now has a fundamental requirement for pthreads, so there
is no compelling reason to retain support for the non-threaded
VNC server. Remove the --{enable,disable}-vnc-thread configure
arguments, and all CONFIG_VNC_THREAD conditionals

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>