Remove unused imports from python files

Review Request #2295 — Created May 31, 2015 and submitted — Latest diff uploaded

dturner-tw, jsirois, zundel
This was done by running Intellij's Optimize Imports tool over the
python files. I then checked the BUILD files against the diff manually.

I reverted a couple of style conflicts with isort (intellij moves
'import sys' to its own section) but other than that this was easy
and took 15-20 mins. The only tedious part was that many BUILD files
use "dependency bags" which meant I often had to grep the entire
module to see if a dep could be removed from the bag or not.

Pull request in the Bugs field.