remove unused imports

In the move to make flake8 tests stricter, we need a code
cleanup earlier.
This removes all unused imports and also enables tests to prevent
them for the future.

This patch also includes the checks on __init__.py files.

Change-Id: I34055803f7c4726682ac6fb95cc5b50ba761fec8
Fixes: bug 1188134
Merge "remove unused imports"