Github doesn't seem to use MP for highlighter tokenization

MagicPython is still used by Linguist, but the actual syntax
hightlighting seems to be driven by something else now.  Thus, remove
the outdated statement from the README.

Closes: MagicStack#225

Author: elprans

README.md

@@ -6,8 +6,6 @@ with [Sublime Text](http://www.sublimetext.com), [Atom](http://atom.io) and
 [Visual Studio Code](http://code.visualstudio.com). It is meant to be a drop-in
 replacement for the default Python package.
 
-We are proud to say that MagicPython is used by GitHub to highlight Python.
-
 **Attention VSCode users**: MagicPython is used as the _default_
 Python highlighter in Visual Studio Code. Don't install it unless you
 want or need the cutting edge version of it. You will likely see no