Remove Grafana attribute and deprecation messages

Remove deprecation messages

• remove Grafana removal deprecation messages
• remove attribute files/gitlab-cookbooks/package/libraries/config/gitlab.rb: attribute('grafana')

Preparation when creating the Issue

• List all the Remove deprecation messages section all the deprecations you want to remove.
• Set the milestone of this issue to next planned required stop, which should be after the milestone when the configuration will be removed.

Preparation when executing the Issue

• Verify that the related configuration got indeed removed in a previous milestone.
• Make sure the MR removing the deprecation message is released on a required stop which is later than the removal of the configuration.