util: drop qemu_fork()

Fortunately, qemu_fork() is no longer used since commit
a95570e3 ("io/command: use glib GSpawn, instead of open-coding
fork/exec"). (GSpawn uses posix_spawn() whenever possible instead)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230221124802.4103554-2-marcandre.lureau@redhat.com>