Removes note on disabled "update group sharing" functionality on gitlab.com

Functionality was previously disabled, temporarily, during 12.8 It has been re-enabled for all instances of gitlab.com

doc/user/group/index.md

@@ -259,7 +259,6 @@ Alternatively, you can [lock the sharing with group feature](#share-with-group-l
 ## Sharing a group with another group **(CORE ONLY)**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/18328) in GitLab 12.7.
-> This feature has been [disabled on GitLab.com](https://gitlab.com/gitlab-com/gl-infra/production/issues/1635).
 
 Similarly to [sharing a project with a group](#sharing-a-project-with-a-group),
 you can share a group with another group to give direct group members access