Remove reference to Grafana service

Remove the following instructions from files/gitlab-cookbooks/monitoring/recipes/default.rb:

# Disable Grafana service.
runit_service 'grafana' do
  action :disable
end

The following discussion from !7078 (merged) should be addressed:

• @balasankarc started a discussion:

  @pursultani Let's open an issue for 16.4 (after the mandatory stop at 16.3), and link it here as a TODO item.