[QOL] Remove deploy blocker labels from issues when we close the checklist

[roryabraham]

Problem

When we close the checklist, it signifies that all current deploy blockers have been resolved or demoted. However, if we don't remember to manually remove the DeployBlockerCash label, they'll appear on the next checklist, which is distracting because they aren't real deploy blockers.

Solution

When we close the checklist, automatically remove the DeployBlockerCash labels from any issues that have them and then comment on the issue.