Replace end-to-end test for deleting a group board by an rspec test

The end-to-end test that deletes a group board is currently in quarantine due to transient failures on CI that are not reproducible locally. Because of this, we have decided to remove it and replace it with a test on a lower level of the test pyramid.

This issue is a follow-up on the following discussion #34936 (comment 237510782).

Cc @felipe_artur.