Remove .border-radius LESS mixin and use border-radius

border-radius is supported without vendor specific prefix in all modern
browsers (https://developer.mozilla.org/en-US/docs/Web/CSS/border-radius).

In many places, the mixin wasn't even used and only old properties with
vendor prefixes were present.

Change-Id: Ib27fb844279a24452b75929ce097ca7e7b80a2e7