PR labeller is wonky

[tcharding]

#615 has C-electrsd but does not touch electrsd.

[jamillambert]

It looks like there was a transient state on June 4th where there was a difference in the PR head to master in electrsd (not in the PR commits themselves), the label was then added. There was then another force push that removed the diff, I guess because of a rebase on master, but the labeller only adds and does not remove labels.

#648 changes the job to also remove the managed labels.