When deleting a group in the Groups dashboard, use `bp_current_user_can()`.

The groups admin frontend also uses `bp_current_user_can()` to determine
whether a user can delete a group.  Thus, this commit mirrors this
capability check in the Groups admin dashboard.

Fixes #5348.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8681 cdf35c40-ae34-48e0-9cc9-0c9da1808c22