Remove legacy service count keys
What does this MR do?
After a conversation with @mpeychet_ https://gitlab.com/gitlab-data/analytics/-/issues/4476#note_365835846, we can remove the legacy service count keys.
projects_slack_notifications_active
and projects_slack_slash_active
are not used on any dashboard anymore. We can remove
them.
Related to https://gitlab.com/gitlab-data/analytics/-/issues/4476, https://gitlab.com/gitlab-data/analytics/-/issues/3241