venus: drop unused sem_feedback_count from vn_queue_bind_sparse_submit_batch

venus: drop unused sem_feedback_count from vn_queue_bind_sparse_submit_batch

The values isn't used later.

Resolves gcc warning:
```
../src/virtio/vulkan/vn_queue.c:1006:13: error: variable 'sem_feedback_count' set but not used [-Werror,-Wunused-but-set-variable]
   uint32_t sem_feedback_count = 0;
```

Fixes: a55d26b566f1 ("venus: add back sparse binding support")

Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23034>