aio: Get rid of qemu_aio_flush()

There are no remaining users, and new users should probably be
using bdrv_drain_all() in the first place.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

main-loop.c

@@ -432,11 +432,6 @@ QEMUBH *qemu_bh_new(QEMUBHFunc *cb, void *opaque)
     return aio_bh_new(qemu_aio_context, cb, opaque);
 }
 
-void qemu_aio_flush(void)
-{
-    aio_flush(qemu_aio_context);
-}
-
 bool qemu_aio_wait(void)
 {
     return aio_poll(qemu_aio_context, true);