... and remove unused SmStructureNode's copy ctor/assignment operator.
This seems desirable according to the old comment in SmNode::Accept().

Change-Id: Ifb51c64ca784f1295374d31cc210cbe3e45aa2f8
Reviewed-on: https://gerrit.libreoffice.org/22801Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Takeshi Abe <tabe@fixedpoint.jp>