Closed paths with 2 points can be created from scratch,
therefore allow them too, if a point is deleted.
Remove now unused member mbIsClosed

Change-Id: Iac2a08df52f734c99c6ac14d9f1a16d8b943d399
Reviewed-on: https://gerrit.libreoffice.org/33654Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>