Remove unused CSS rules. These were found by using used-css-classes [1]
and find-unused-css-classes [2]:

  used-css-classes h/templates/client/*.html > used-classes.txt
  find-unused-css-classes used-classes.txt build/styles/app.css > unused-css-classes

And then manually checking classes in `unused-css-classes` to verify
that the classes were not referenced in first or third-party code, since
the tools only extract used class names from templates.

[1] https://github.com/robertknight/used-css-classes
[2] https://github.com/robertknight/find-unused-css-classes