Remove all unused variables.
Unused variables were found via pylint.
There are a few types of circumstances where we have unused variables,
so there are a few types of transformations:
1. variables that are simply unused, e.g.
a = f() => <delete>
2. functions that return tuples but only some arguments are used, e.g.
a, b = f() => a, _ = f()
3. functions that have side effects and also a return value, e.g.
a = do_something() => do_something()
4. exception variables that are unused
except Exception as e: => except Exception:
Review-Url: https://codereview.chromium.org/2136793002
Cr-Commit-Position: refs/heads/master@{#406887}
39 files changed
