Remove 'new subscription' page from dotcom

Created by: ryphil

In 2021, the decision was made to stop allowing self-service sign-ups of the Teams plan on dotcom. At that time, we removed the 'new subscriptions' button from the subscriptions page on dotcom as a stop-gap to prevent users from signing up on this new plan.

Recently, a team discovered that if they navigate to the URL (ex: https://sourcegraph.com/users/ryphil/settings/subscriptions/new), the page and functionality still exists. Using this method, they were able to self-service purchase an outdated plan of Sourcegraph. This new subscription page returns 404 on k8s and S2. More details in the Slack thread.

Definition of success:

• The new subscription page is completely removed from dotcom