Remove Promotion check summary from Slack output

Currently, the Slack message shows a top-level status ("go / no-go"), then a summary, then the details of each check.

In practice, we only have one or two active incidents, and possibly one or two change requests. It's never so many that we can't simply eyeball a count from the full description.

This change removes the summary for each check, leaving just the top-level status and the details.

Before:

After: