Remove group permissions UI from secrets manager settings

What does this MR do and why?

We are deprecating group permissions for the secrets manager. This implements Step 2 of #623457 (comment 3783831147), where we update the UI to remove the group tab/table, and modal/form for creating group permissions.

This MR should only be merged once Step 1 changes (!254051 (merged)) are merged.

References

Implements the frontend part of #623457 (closed)

Part of the UI improvements work for permissions: #623352

Screenshots or screen recordings

Before (Table)	Before (Modal)	After

How to set up and validate locally

Setting up the secrets manager

1. Upload a Premium license (or above).
2. Set up the GDK with OpenBao: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/openbao.md
3. Enable the following feature flags: secrets_manager, group_secrets_manager, secrets_manager_instance_enrollment.

Verifying the feature

1. Go to the Admin settings for the secrets manager (/admin/application_settings/general#js-secrets-manager-instance-enrollment-settings) and enable the toggle.
2. In your group, visit the secrets manager settings (/-/edit#js-permissions-settings, at the bottom of the section) and enable the toggle.
3. The permissions table should show up. Verify that:
   • The Group tab does not exist
   • When selecting from the dropdown to add new permissions, the Group option does not exist

MR acceptance checklist

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