If the copy c'tor of ProcessorSelection was actually used,
assigning the XMLProcessorSelection
processors = other.processors;
would lead to duplicate free() of the XMLNode*
XMLProcessorSelection would need a dedicated copy c'tor that
duplicates allocates a new XMLNode.
see also #10 at https://www.viva64.com/en/b/0540/
Anyway, the copy c'tor and assignment is never used. This commit makes
this explicit.