!scr do not remove tags (github) that are not set on an issue

Since the updated github issues views, it shows all "removed tags",
events in the issue even if the removed tag was never set on the issue.
So this script would always show up as "removed validating,needs-attention"
Even if the issue never had the label needs-attention set to it,
which is confusing visually.

This change introduces that we only remove tags that actually are set on
the issue.
Oh well, then. :)