Removed serbian translation for 2.0, too

Refs https://github.com/cakephp/docs/pull/1948

Turns out the serbian translation isn't kept up to date for 2.0 either.
No wonder when the initial maintainer  "Aleksa-Savic" disappeared.

The commits in this tree are not many:
https://github.com/cakephp/docs/commits/master/sr