It can really be a pain deleting a lot of posts. Thankfully there's bulk edit here on WordPress.
Then if you want to delete all the posts under "uncategorized", click that drop-down menu "view all categories" and select "uncategorized". Click "Filter"
Tick the box on the right side of "Title" so that all the boxes for each titles have checks.
Bulk action > Move to trash> Apply.
1000 won't work for screen options but 999 do so you can do it 999 at a time. :)
I hope this helps!