Revome useless MathJax files added in Ib7f2e4b8eb7250ea4fc24aac3cedf196211f38c2

These files were added in but the change was totally unrelated to the
AsciiMath issue mentioned there and moreover this commit bypassed review.
These entity references are useless for LaTeXML
(which does not generate entity names), for the client-side MathJax
(which does not use MathML input) and for mathoid (which uses
phantomjs=webkit and thus already parses the HTML5 entity names correctly).
Finally, these files are likely to disappear in future MathJax versions.
So let's remove these files until it is clear what the intention was.

Change-Id: Iee875632411b98d1e86d3bb10f69ae3797105de7