Remove duplicate group boards doc

The feature is already documented in https://docs.gitlab.com/ee/user/project/issue_board.html#group-level-issue-boards

doc/user/group/issue_boards.md

-# Group issue board
-
-> Introduced in GitLab 10.0.
-
-Group issue boards help users manage teams in organizations
-that have a team-centered or product-centered approach which spans
-many projects.
-
-![Group Issue Board](img/group_issue_board.png)
-
-The design and functionality are the same as [project boards](../project/issue_board.md) with some small differences:  
-
-* In a group board all issues within group projects will be displayed 
-on backlog or closed lists. Users don't need to filter them by project on search bar.
-
-* Group boards can only have [group milestones](../project/milestones/index.md#creating-a-group-milestone) associated with them.
-
-* Only group [labels](../project/labels.md) can be used to create lists inside each board.
-
->**Note:**
-
-Only issues in immediate child projects of the group are available in the group board. Issues in further descendant subgroups are not shown in the group board.
-