Remove stale Mtx link.

Also, clean up links to Simbody, now that we are using a Simbody tag file. That is, those Simbody links at the bottom do not link to Simbody documentation online; they link to the user's local copy of Simbody. Maybe that is not a good idea, because the person might not have built Simbody documentation locally?

Before:


After:
![screenshot from 2014-08-04 15 26 24](https://cloud.githubusercontent.com/assets/846001/3804460/9d5af06c-1c26-11e4-82d7-7e6575023385.png)