Remove unused "differential_equations.md" and "math.md".

[june128]

No description provided.

[june128]

This files don't seem to be used or serve any purpose.

[leios]

I use math.md regularly. In order to get mathjax to work, I need that set of commands somewhere on the chapter, so I kept the set of commands in a central directory to copy from. There might be a way to directly import math.md to other chapters instead of just copying it's contents.

If we delete differential_equations.md, I'll just end up adding it back later when we need it, but we can delete it for now, if you want to clean up the directory structure.

[june128]

Ok, the math.md is there again. The PR is just now a minor organizational change.
You can either delete or merge it; what you like better.

[leios]

This file should be deleted. I don't know when we'll work on Diff. Eq's again.