fix(css): rename and remove invalid CSS utils

BREAKING CHANGE: this removes a few CSS utils that were incorrectly
named in favor of new ones that follow the naming conventions.
If you were using any of the removed utils, please migrate to the new
ones:

* gl-flex-sm-wrap was replaced with gl-sm-flex-wrap.
* gl-justify-content-md-start was replaced with
  gl-md-justify-content-start.
* gl-pt-md-2 was replaced with gl-md-pt-2.
* gl-mt-md-n2 was replaced with gl-md-mt-n2.