coroutine: remove GThread implementation

The GThread implementation is not functional enough to actually
run QEMU reliably. While it was potentially useful for debugging,
we have a scripts/qemugdb/coroutine.py to enable tracing of
ucontext coroutines in GDB, so that removes the only reason for
GThread to exist.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>