Removed serbian translation

I has not received any specific commits for the 3.0 branch apart from global edits and follow ups:
https://github.com/cakephp/docs/commits/3.0/sr

The only seemingly specific commits are actually commits that were initially posted towards 2.0:
 https://github.com/cakephp/docs/pull/1056

 Also the user "Aleksa-Savic", who posted those commits, has deleted his GitHub account as it seems.