Drop unused method in TreeInfo
Drop unused method in TreeInfo

I overlooked that we actually have a `closure` extractor, which would
do the job of `isClosure`, if we ever needed it.

dotty-staging/dotty,sjrd/dotty,sjrd/dotty,sjrd/dotty,som-snytt/dotty,som-snytt/dotty,lampepfl/dotty,dotty-staging/dotty,lampepfl/dotty,som-snytt/dotty,lampepfl/dotty,sjrd/dotty,lampepfl/dotty,som-snytt/dotty,dotty-staging/dotty,dotty-staging/dotty,som-snytt/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty