Help: Update 3.1 release notes for dropping of FindITK and FindVTK

The changes made by commit v3.1.0-rc1~46^2~1 (FindITK: Drop this ancient
compatibility module, 2014-10-02) and commit v3.1.0-rc1~46^2 (FindVTK:
Drop this ancient compatibility module, 2014-10-02) need to come with
release notes explaining that the modules were dropped.  Also remove a
release not related to an update made to FindVTK that was never
released.
Merge topic 'doc-FindITK-FindVTK-relnotes' into next

1550d97d Help: Update 3.1 release notes for dropping of FindITK and FindVTK