Remove references of mapred.child.ulimit etc. since they are not being used any more

Courtesy Philip Su, we found that (mapred.child.ulimit, mapreduce.map.ulimit, mapreduce.reduce.ulimit) were not being used at all. The configuration exists but is never used. Its also mentioned in mapred-default.xml and templates/../mapred-site.xml . Also the method getUlimitMemoryCommand in Shell.java is now useless and can be removed.