Remove namespaces ineligible for trial from subscription list in the trial flow
## Problem
We are misleading users by offering the ability to attach trials to groups or username that are already in an active trial or which currently have an active Gold-level subscription. We are following back by displaying an error message, but the whole process can be avoided and simplified.
## Input Data
* Current flow - [New user from Marketing website](https://app.mural.co/t/gitlab2474/m/gitlab2474/1580984258623/a815e52decef6141307b634da65fbcd5242a48e8)
## Proposed Solution
* For users that already have a trial on a group or a username, remove this items from the list. Only suggest to create a group or to attach the subscription on a free group.
* For users that have trials on group only, decide wether we should potentially remove the ability to attach it to a username based on [this experiment](https://gitlab.com/gitlab-org/growth/product/issues/194) results.
## How de we measure success
* Observe a decrease in time to completion
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