Remove unmaintained russian translation

The most current russian translation is from Dec 19, 2013:
https://github.com/cakephp/docs/commits/master/ru

Actual russian translation commits:
Dec 19, 2013
Dec 12, 2011
Dec 11, 2011
Dec 10, 2011
Dec 9, 2011
Nov 20, 2011
Nov 19, 2011
Nov 13, 2011
Nov 12, 2011
Nov 11, 2011
Nov 10, 2011

See yourself:
http://book.cakephp.org/2.0/ru/contents.html