Remove obsolete trial code

Proposal

Let's investigate whether the /trials/new action and its related views can be removed. If they aren't used anymore, removing all this related code should simplify the trials code in CustomersDot, which is pretty complex at the moment.

Follow-up

The following discussion from !3743 should be addressed:

• @jagood started a discussion: (+2 comments)

  I'm not sure what this function is for. I only see it used here: https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/e65123726c8ffb93bad38783f00ecdb2bf467b73/app/views/trials/_simple_gitlab_com_trial_form.html.haml#L16

  Just guessing from the heading of that file, "Free Trial of GitLab.com Gold", maybe this is all unused code at this point?

We should also look at removing app/views/trials/_gitlab_com_trial_form.html.haml as well as it also refers to a Gold Trial.

This should also take care of the work mentioned in #2782 (closed) related to removing these views:

• app/views/trials/new.html.haml
• app/views/trials/_simple_gitlab_com_trial_form.html.haml