Bug 875305 - Remove slow CSS selectors for WebVTT. r=bz, a=akeybl

Bug 833388 added rules to the default stylesheet for styling WebVTT
cues displayed in an html video element. The code to drive this
didn't land for Firefox 22, and the rules don't follow best practices
for performance and could slow down page rendering.

This commit just removes the rules since they're not used.