spice: drop obsolete iothread locking

We don't use qemu internals from spice server context any more.
Thus we don't also need to grab the iothread mutex from spice
server context.  And we don't have to temporarely release the
lock to avoid deadlocks.  Drop all the calls.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
