Remove ldap_settings_unlock_groups_by_owners feature flag

What

Remove the :ldap_settings_unlock_groups_by_owners feature flag

Owners

• Team: ~"group::access"
• Most appropriate slack channel to reach out to: #g_manage_access
• Best individual to reach out to: Sebastián Arcila Valenzuela @sarcila

### What are we expecting to happen?

When the admin of the instance allow owners to manage ldap sync, they should be able to manually enable the option to add/remove users outside the defined ldap group

What might happen if this goes wrong?

We don't expect major impact, the feature flag is mostly used to be able to split the work and be able to merge the different parts, more than reducing the risk of it

Owners won't be able to remove users that they added or reset to just using ldap sync

• Ensure that documentation has been updated
• Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry

Related to #1793