Remove -webkit-backface-visibility on skew mixin

This rule is causing pixelated edges on iOS, even though that's what it was supposed to fix (#5319). Removing it makes everything smooth across all OS's, browsers, and devices.

Before:

![before](https://f.cloud.github.com/assets/115911/218052/902aa40c-84fe-11e2-9245-494c9d3e0567.png)

After:

![after](https://f.cloud.github.com/assets/115911/218054/951dc26e-84fe-11e2-9fd0-df3a9014e8ea.png)