Page on front option disappears when no pages are available

[markjaquith]

Edge case, but if you have WP set to display a page on the front page, and you delete that page, and it was the last page, WP will maintain the option, but prevent you from changing it.

When deleting pages, we should check to see if the page was the page_on_front page, and if so, delete the page_on_front and show_on_front options.

Looking into the code, I have found that this functionality already exists. Testing also shows that this works as expected. If the page is listed as the front page the option is removed and the page for posts is reset also.