SI-6916 makes FlatHashTable#remove a Boolean not Option[A]

Makes FlatHashTable#remove return a boolean instead of Option[A].
Updates HashSet accordingly. Adds a test to make sure remove
works as advertised.
Merge pull request #1847 from JamesIry/SI-6916_master

SI-6916 makes FlatHashTable#remove a Boolean not Option[A]
Merge branch 'master' of https://github.com/scala/scala into parse-java7