Remove outdated/inexistent vendor prefixed properties.

Properties that are supported without any prefix in all major browsers:

* `background-size`
* `box-shadow`
* `opacity` - (IE 8 requires the `filter: alpha` property)
* `border-radius`
* `box-sizing`
* `transition`

Properties that are stable in some browsers, but require specific prefixes for
recent or current versions:

* `animation`
* `transform`
* `linear-gradient`