Labeller no longer removes outdated labels from PRs

[krassowski]

As noted in #16228 (comment) the labeller did not remove labels which are no longer needed. I also saw that recently @JasonWeill had to manually remove dozens of labels on #16207.

This was not the case previously. It appears to be the result of a recent upgrade to labeller action v5 which fixed a bug in sync-labels setting. Namely, prior to v5 the default was de-facto "true" (it was false it was incorrectly parsed to true).

I think the old behaviour was better.