Remove a case that is not negative examples in the capybara sheet (#2122

)

The above case is a new Bad case added by #1798 . It is designated as Bad due to performance issues, but it is not actually a Negative example. In practice, the following would be the same test:

```ruby
expect(page).to have_button('Save')
!expect(page).to have_button('Save')
```

This is not an example that will appear on the capybara cheat sheet, because it is a problem with how RSpec is written. I think it should be removed because it creates confusion.

capybara.md

@@ -119,15 +119,15 @@ In RSpec, you can use `page.should` assertions.
 ### About negatives
 
 ```ruby
-expect(page).to have_no_button('Save')   # OK
-```
-```ruby
-expect(page).not_to have_button('Save')  # OK
+expect(page).to have_no_button('Save')
 ```
+
 ```ruby
-!expect(page).to have_button('Save')  # Bad
+expect(page).not_to have_button('Save')
 ```
 
+The two above statements are functionally equivalent.
+
 ## RSpec
 
 ### Matchers