SI-8774 mutable LinkedHashMap/LinkedHashSet nodes' link fields should be nulled out when removed

[szeiger]

https://issues.scala-lang.org/browse/SI-8774

[adriaanm]

see 2.10.5 fix scala/scala#3911

[varming]

I just submitted a PR scala/scala#5295 that handles all the link/next links in the various hash sets and maps.