SI-8264 scala.collection.immutable.HashSet#- returns broken Set

Was an error in HashSet's handling of removal of an element when a HashTrieSet should turn into a HashSet1.

Also slightly modified HashMap's filter0 to more closely match HashSet (by adding the same comment).
Merge pull request #3527 from Ichoran/issue/8264

SI-8264 scala.collection.immutable.HashSet#- returns broken Set