Remove trivial cases of unused variables (3)

Kill some of the variables marked as unused by flakes8. This should
allow to enable F841 check in the future.
Only trivial cases with no function calls and obviously pure functions
(like datetime.now(), or len()) are cleaned up here.

Part 3, split to reduce conflicts.

Change-Id: I7b8e806ddd7b0e077243d4e31de140cd08fe0155
Merge "Remove trivial cases of unused variables (3)"