aio: remove process_queue callback and qemu_aio_process_queue

Both unused after the previous patch.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/linux-aio.c b/linux-aio.c
index 15261ec..fa0fbf3 100644
--- a/linux-aio.c
+++ b/linux-aio.c
@@ -214,7 +214,7 @@
         goto out_close_efd;
 
     qemu_aio_set_fd_handler(s->efd, qemu_laio_completion_cb, NULL,
-        qemu_laio_flush_cb, NULL, s);
+        qemu_laio_flush_cb, s);
 
     return s;