Fix issue with merge-pr.py script
The commented line shown produces an error on fresh clones of pandas, and is not needed AFAICT.

this gave me an error after it pushed the merged branch to master

it said: `Would you like to pick %s into another branch?`

and I said `Y`, at which point `latest_branch` was undefined. Not really even sure what that's supposed to do. I presume it would have cherry-picked to my local copy of master, as the remote master was already updated. 

The script exited and all was ok.

What is the purpose of that section?
 You say no to that. We can also remove that section
 @jreback I removed the cruft. If looks good merge away. If we find ourselves doing much backporting to maintenance branches in the future we can add some tools for that
 merged. thanks.
