Remove feature flag for new trial sign up flow
The new trials is behind the FF `improved_trial_signup`. The feature flag was originally implemented in case we needed to roll back, while we still have some outstanding issues ([documented here](https://gitlab.com/groups/gitlab-org/-/epics/1902)) the overall flow has been successful. We should remove this feature flag and ensure 100% of traffic goes to the new trial sign up flow per recommendations on [Cleaning up of Feature Flags](https://docs.gitlab.com/ee/development/feature_flags/controls.html#cleaning-up).
# History
- The flag was enabled on 23rd of October, 2019.
- The links to the new trials was updated on 29th of October, 2019 (the day we opened to customers).
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