Remove trivial cases of unused variables (1)

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 1, split to reduce conflicts.

Change-Id: I82854349574ec4bcb9336aba626eefdaed81a8c8
Merge "Remove trivial cases of unused variables (1)"