[Feature flag] Remove subscribable_license_banner

What

Remove feature flag :subscribable_license_banner.

See: gitlab-org/growth&3 (closed)

Note

1. this was originally added in development as :subscribable_banner_license.
2. this refers to self-managed (license), there is another flag for SaaS (:subscribable_subscription_banner) #230858 (closed)
3. tracking this flag as it has been part of the release with ::Feature.enabled?(:subscribable_license_banner, default_enabled: true) and will need to be cleaned up.

TODO

1. Add docs per https://docs.gitlab.com/ee/development/documentation/feature_flags.html#features-that-became-enabled-by-default
2. See also comment about test coverage below.

The following discussion from !30304 (merged) should be addressed:

• @pslaughter started a discussion:

  suggestion (non-blocking: The fact that this bug existed with a green pipeline suggests some missing test coverage. IMO, that doesn't need to block this MR, but would be nice to cover in a follow-up.

Owners

• Team: group::retention
• Most appropriate slack channel to reach out to: #g_retention
• Best individual to reach out to: @jayswain

Expectations

### What are we expecting to happen?

What might happen if this goes wrong?

What can we monitor to detect problems with this?

Beta groups/projects

If applicable, any groups/projects that are happy to have this feature turned on early. Some organizations may wish to test big changes they are interested in with a small subset of users ahead of time for example.

- gitlab-org/gitlab project - gitlab-org/gitlab-com groups

• ...

Roll Out Steps

• Enable on staging
• Test on staging - [ ] Ensure that documentation has been updated - [ ] Enable on GitLab.com for individual groups/projects listed above and verify behaviour - [ ] Coordinate a time to enable the flag with #production and #g_delivery on slack. - [ ] Announce on the issue an estimated time this will be enabled on GitLab.com - [ ] Enable on GitLab.com by running chatops command in #production
• Cross post chatops slack command to #support_gitlab-com (more guidance when this is necessary in the dev docs) and in your team channel
• Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry
• After the flag removal is deployed, [clean up the feature flag](https://docs.gitlab.com/ee/development/feature_flags/controls.html#cleaning-up) by running chatops command in #production channel