[nomerge] SI-8899 Revert "SI-8627 make Stream.filterNot non-eager"

This reverts commit 9276a1205f74fdec74206209712831913e93f359.

The change is not binary compatible, See discussion on SI-8899. Making
filterImpl non-private changes its call-sites (within TraversableLike)
from INVOKESTATIC to INVOKEINTERFACE. Subclasses of TraversableLike
compiled before this change don't have a mixin for filterImpl.