Remove `cursor: not-allowed` styles

Per discussion on https://github.com/twbs/bootstrap/issues/22222#issuecomment-292819959

> browsers don't normally use the "not-allowed" cursor in these circumstances, so it makes BS a bit opinionated here. (and, at least on Windows, the "not-allowed" cursor looks rather fugly with its red colour compared to other regular mouse pointers)
>
> ![capture](https://cloud.githubusercontent.com/assets/895831/24841663/1f9bbd46-1d81-11e7-9e96-54502b731edf.PNG)

Closes https://github.com/twbs/bootstrap/issues/22222