Remove libtess.js

This removes the modified version of [libtess.js](https://github.com/brendankenny/libtess.js) from our repository.  The same commits are preserved in the openlayers/libtess repository.

If we need libtess.js in the future, these are some options:

 1) add the existing [libtess.js](https://github.com/brendankenny/libtess.js) as a dependency that is pulled in at install time (this assumes any changes we need can be pushed to that repo)
 2) maintain a fork of [libtess.js](https://github.com/brendankenny/libtess.js) that includes any changes that are not acceptable to the original project and add that as a dependency
 3) copy the modified version back into our repository

My preferences are indicated by the order of the options above.

Fixes #2271.