Remove bin/qa since it's redundant with exe/gitlab-qa

The following discussion from !231 should be addressed:

• @ddavison started a discussion: (+1 comment)

  This also makes exe/gitlab-qa redundant with bin/qa so we don't have to maintain duplicate scripts. This is done by moving the code from bin/qa to its own class, which also makes it easier to unit test.

  because of this redundancy.. should we think about removing bin/qa or should we leave this in there for now until the next major version

I think most users of gitlab-qa would be using the gem, so I'm not sure bin/qa is used anywhere other than in CI in this project.

So it's probably safe to remove.