MathJax: Remove our RGB extension again, added in MathJax 2.1

Turns out that with the upgrade to 2.1 this is now included in core
MathJax, so we don't need the extension anymore.
http://docs.mathjax.org/en/latest/whats-new-2.1.html

Change-Id: I653c05aaf6d94851fdf6b1649d2d5c7477d46b7e
Merge "MathJax: Remove our RGB extension again, added in MathJax 2.1"