[SPARK-1041] remove dead code in start script, remind user to set that in spark-env.sh

the lines in start-master.sh and start-slave.sh no longer work

in ec2, the host name has changed, e.g.

ubuntu@ip-172-31-36-93:~$ hostname
ip-172-31-36-93

also, the URL to fetch public DNS name also changed, e.g.

ubuntu@ip-172-31-36-93:~$ wget -q -O - http://instance-data.ec2.internal/latest/meta-data/public-hostname
ubuntu@ip-172-31-36-93:~$  (returns nothing)

since we have spark-ec2 project, we don't need to have such ec2-specific lines here, instead, user only need to set in spark-env.sh