LockRoster.modify: no KeyError if element was already gone, fixes #7937 #7941
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The intention of LockRoster.modify(key, REMOVE) is to remove self.id.
Using set.discard will just ignore it if self.id is not present there anymore.
Previously, using set.remove triggered a KeyError that has been frequently seen in tracebacks of teardowns involving Repository.del and Repository.exit.
I added a REMOVE2 op to serve one caller that needs to get the KeyError if self.id was not present.
Thanks to @herrmanntom for the workaround!