CLN: Remove "flake8: noqa" from more files
Another round of house-cleaning that builds off #15842.

xref <a href="https://github.com/pandas-dev/pandas/issues/12066#issuecomment-172285473">#12066  (comment)</a> : the issue remains unresolved, but it does not seem entirely necessary to disable style-checking on the entire file for that IMO.